RFR: 7903780: NPE in new code for LIBRARY.properties

[Jonathan Gibbons]

As well as the NPE, the following additional minor issues were fixed:

`--source` is a recent addition for `javac` options; older versions of `javac` still require `-source N`

`--enable-preview` is required at runtime if any library uses preview features.
For now, I just moved `usesLibraryCompiledWithPreviewEnabled` from `CompileAction` up into `Action`, so that it can be used from `MainAction`. We might want to consider merging the merging into `RegressionScript.enablePreview`, but that can be done when we clean up the use of `LIBRARY.properties`, caching the results in `LibLocn`.

We should also investigate the difference between `libLocn == null` and `libLocn.isTest()` 

After this, all self-tests pass, including the recently new tests for `LIBRARY.properties`.

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Commit messages:
 - CODETOOLS-7903780: NPE in new code for LIBRARY.properties

Changes: https://git.openjdk.org/jtreg/pull/213/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=213&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903780
  Stats: 23 lines in 3 files changed: 12 ins; 8 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/213.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/213/head:pull/213

PR: https://git.openjdk.org/jtreg/pull/213