Resolve symbolic links before comparing the test JDK with Maven's JDK

[cwegener-79]

ForkStarter#isForkJvmDifferentFromBuildJvm compares the JDK home of the tests with Maven's own JDK home using plain File#equals:

private boolean isForkJvmDifferentFromBuildJvm() {
    File testsJdkHome = forkConfiguration.getJdkForTests().getJdkHome();
    return testsJdkHome != null && !testsJdkHome.equals(toJdkHomeFromJre());
}

Two different spellings of the same directory therefore count as two different JDKs. This is easy to hit on CI machines, where the JDK is often reached through a symbolic link or an automounted path.

Since #3444, that comparison decides whether Surefire starts an extra short-lived fork to list the test classes. When both paths refer to the same JDK, that fork is pure overhead: one additional JVM startup per module, for every build.

What this PR does

Canonicalizes both paths before comparing them, falling back to the absolute path when the file system cannot be queried (unreadable directory, missing path).

Verification

• New unit tests in ForkStarterTest cover relative segments, a symbolic link (skipped via assumption on platforms that do not allow creating one) and two genuinely different directories.
• Checked manually on Linux with a project using forkCount=4 and <jvm> pointing at a symlink to the running JDK:
  • before: an extra fork is started and target/surefire/surefire-discovered-tests*.txt is written,
  • after: neither happens, and the tests run exactly as before.

No behaviour changes when the test JDK really is a different one - the listing fork is still used in that case.

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