Ensure title exported in SCRUT_TEST contains only printable characters

Summary:
This CL changes the value of SCRUT_TEST to contain <file-name>:<line-number>

# What?

- The value of `SCRUT_TEST` now contains the file-name followed by the line number (e.g. `some/test.md:123`) instead of the file-name followed by the title

# Why?

The filtering that Scrut uses to make sure some internal variables are passed between executions only works with single-line values. The newly introduced SCRUT_TEST variable was added to that filter list, but it could contain multi-line values (i.e. when a Scrut test title contained multiple lines). This resulted in invalid `state` files that broke subsequent Scrut tests (see failing tests in MSDK, e.g. D60034934)

Reviewed By: abesto

Differential Revision: D60039617

fbshipit-source-id: e99279c250e8a711d4c51f99c12e1120d702a329

selftest/cases/environment.md

@@ -74,5 +74,5 @@ GREP_OPTIONS: ''
 
 ```scrut
 $ echo "SCRUT_TEST: $SCRUT_TEST"
-SCRUT_TEST: .*[/\\]selftest[/\\]cases[/\\]environment\.md::Haz SCRUT_TEST (regex)
+SCRUT_TEST: .*[/\\]selftest[/\\]cases[/\\]environment\.md:76 (regex)
 ```

src/executors/stateful_executor.rs

@@ -127,7 +127,11 @@ impl Executor for StatefulExecutor {
             testcase.config.timeout = timeout;
             testcase.config.environment.insert(
                 "SCRUT_TEST".into(),
-                format!("{}::{}", context.file.to_string_lossy(), testcase.title),
+                format!(
+                    "{}:{}",
+                    context.file.to_string_lossy(),
+                    testcase.line_number
+                ),
             );
 
             // run the execution, using the shared state directory

website/docs/advanced/specifics.md

@@ -119,7 +119,7 @@ Scrut sets a list of environment variables for the execution. These are set _in
 - `TESTFILE`: contains the name of the file that contains the test that is currently being executed
 - `TESTSHELL`: contains the shell that in which the test is being executed in (default `/bin/bash`, see `--shell` flag on commands)
 - `TMPDIR`: contains the absolute path to a temporary directory that will be cleaned up after the test is executed. This directory is shared in between all executed tests across all test files.
-- `SCRUT_TEST`: contains the path to the test and the title, separated by double colons (e.g. `some/test.md::Some Title`). *This variable is recommend to use when deciding whether an execution is within Scrut.* Note: **the title is provided as given and therefore can contain spaces**!
+- `SCRUT_TEST`: contains the path to the test and the line number, separated by a colon (e.g. `some/test.md:123`). *This variable is recommend to use when deciding whether an execution is within Scrut.* Note: **the title is provided as given and therefore can contain spaces**!
 
 ### Common (linux) environment variables