[SPARK-32408][BUILD] Enable crossPaths back to prevent side effects

### What changes were proposed in this pull request?

This PR proposes to enable `corssPaths` back for now to match with the build as it was.
It still indeterministically doesn't run JUnit tests given my observation, and this PR basically reverts the partial fix from apache#29057.

See also apache#29205 for the full context.

### Why are the changes needed?

To prevent the side effects from crossPaths such as SPARK_PREPEND_CLASSES or tests that run conditionally if the test classes are present in PySpark.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

Manually tested:

```bash
build/sbt -Phadoop-2.7 -Phive -Phive-2.3 -Phive-thriftserver -DskipTests clean test:package
./python/run-tests --python-executable=python3 --testname="pyspark.sql.tests.test_dataframe QueryExecutionListenerTests"
```

Closes apache#29218 from HyukjinKwon/SPARK-32408-1.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

project/SparkBuild.scala

@@ -1029,8 +1029,6 @@ object TestSettings {
     // Show full stack trace and duration in test cases.
     testOptions in Test += Tests.Argument("-oDF"),
     testOptions in Test += Tests.Argument(TestFrameworks.JUnit, "-v", "-a"),
-    // Required to detect Junit tests for each project, see also https://github.com/sbt/junit-interface/issues/35
-    crossPaths := false,
     // Enable Junit testing.
     libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % "test",
     // `parallelExecutionInTest` controls whether test suites belonging to the same SBT project