Test `execute()` in StandaloneTests asserts matching of lines between
expected standard output and actual output lines.  It also passes the
same actual lines (but in a `String` form) as a message to method
assertLinesMatch, which takes three arguments.  Same situation for
standard error.

Drop this confusing argument from the two calls to assertLinesMatch and
let class `AssertLinesMatch` construct the error message for us.

Some tests in StandaloneTests provide concatenation of standard output
and standard error as a message for the assertion about exit code, while
other tests provide only standard output.  Some use method
de.sormuras.bartholdy.Result#getOutput while others use method
de.sormuras.bartholdy.Result#getOutputLines.

Unify these approaches by replacing the messages with a supplier that
gives a) a human-readable message about exit codes, and b) both standard
output and standard error.

Problem
-------
Tests that claim by their name to run on Java 8 don't actually run on
Java 8.  This can be clear from the output for tests that add option
`--show-version` to the arguments and _don't_ fail -- they all print the
version for the current JDK.

The tool `java` of JDK 8 does _not_ have the option `--show-version`.
The actual option that exists in JDK 8 has fewer hyphens, as per
documentation of Java 8 [1]:

    -showversion

    Displays version information and continues execution of the application.
    This option is equivalent to the `-version` option except that the latter
    instructs the JVM to exit after displaying version information.

And when I actually run Java 8 binary with the incorrect option
`--show-version` used by the affected tests, I get:

    $ /usr/lib/jvm/java-8-openjdk-amd64/bin/java --show-version
    Unrecognized option: --show-version
    Error: Could not create the Java Virtual Machine.
    Error: A fatal exception has occurred. Program will exit.

The option `--show-version` was only added in Java 9 [2].

Explanation
-----------
In actuality, the tests are being run on whatever the current JDK is.
These tests create an instance of class `de.sormuras.bartholdy.tool.Java`,
which importantly has the following method:

    @OverRide
    public Path getHome() {
      return Bartholdy.currentJdkHome();
    }

When the `bartholdy` library creates the process, class `AbstractTool`
does the following:

    protected Path createPathToProgram() {
      return getHome().resolve("bin").resolve(getProgram());
    }

The string `"java"` returned from method `getProgram` of class `Java`
gets resolved against `Bartholdy.currentJdkHome()`.

As far as I can tell, the library doesn't promise to look up the `java`
binary in the `JAVA_HOME` supplied in the environment.  In fact, just
before consuming library user's environment, method `run()` of class
`de.sormuras.bartholdy.tool.AbstractTool` puts the current JDK as
`JAVA_HOME` into the environment to correspond to the behavior of class
`de.sormuras.bartholdy.tool.Java` described above:

    builder.environment().put("JAVA_HOME", Bartholdy.currentJdkHome().toString());

The issue has been present since commit [3] where these tests were
introduced.

Fix
---
Fix affected tests to run them under actual Java 8 by overriding method
`de.sormuras.bartholdy.tool.Java#getHome`.  Replace erroneous option
`--show-version` with `-showversion`.

To make tests executeOnJava8() and executeOnJava8SelectPackage() see the
class files, update test compile() to use option `--release 8`.  Because
compiling to release 8 is deprecated, add a linter option to disable the
warning to make compile() pass.

Because option `-showversion` of Java 8 behaves slightly differently to
option `--show-version` of later versions of Java, prepare two new files
for expected stdout and stderr: expected-out-java8.txt and
expected-err-java8.txt, which are similar to existing files
expected-out.txt and expected-err.txt, but have different layout of
fastforward lines "JAVA VERSION" and "TREE".

Footnotes
---------
[1] "Java Platform, Standard Edition Tools Reference", "java"
    https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html
    https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html
[2] https://docs.oracle.com/javase/9/tools/java.htm

    > `--show-version` or `-showversion`
    >
    > Displays version information and continues execution of the application.
    > This option is equivalent to the `-version` option except that the latter
    > instructs the JVM to exit after displaying version information.
[3] c62cd6a (Fix package path computation in `ClasspathScanner`,
    2021-05-12) from junit-team#2613