RFR: 8249095: tools/javac/launcher/SourceLauncherTest.java fails on Windows

[Jan Lahoda]

@rwestberg  prepared a script that runs various tests in GitHub Actions. But, sadly, with a recently proposed fix for
Windows handling (see https://github.com/openjdk/jdk/pull/437), the SourceLauncherTest is failing (the
testNoSourceOnClassPath sub-test): https://github.com/rwestberg/jdk/runs/1188397548

I don't think this has much to do with javac (or source launcher) itself - the test fails while it tries to write the
test source file, before the source launcher is invoked. So this patch tries to: a) avoid doing that; b)  improve
ToolBox to check and reject file names that are reserved.

Note that testHelloWorldWithAux uses the name "Aux" intentionally, and it passes because it never actually writes file
with name "Aux" - the source file will be (I believe) named "HelloWorld", and while it will contain also a class named
"Aux", the classfile will only ever exist in memory, and won't be written as a file, so will not cause the issue.

The use of name "Aux" in the testNoSourceOnClassPath test does not seem to be necessary, and the issue happens in the
test set-up, not while doing the actual test, so it seems fine to change the name.

@jonathan-gibbons, could you please take a look? Thanks!

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Commit messages:
 - 8249095: tools/javac/launcher/SourceLauncherTest.java fails on Windows

Changes: https://git.openjdk.java.net/jdk/pull/456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249095
  Stats: 29 lines in 2 files changed: 21 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/456.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/456/head:pull/456

PR: https://git.openjdk.java.net/jdk/pull/456