RFR: 7903166: jtreg crashes trying to compile .jasm with enablePreview=true [v2]

[Evgeny Nikitin]

> The bug: [CODETOOLS-7903166](https://bugs.openjdk.java.net/browse/CODETOOLS-7903166)
> 
> A small fix targeting crashes that occurs when jtreg compiles non-javac files with enablePreview. Prior the fix, the code crashed trying to insert arguments into javacArgs list using negative index.
> 
> Testing: test/preview folder self-tests.

Evgeny Nikitin has updated the pull request incrementally with one additional commit since the last revision:

  Test for runJavac variable prior adding --enable-preview

-------------

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/77/files
  - new: https://git.openjdk.java.net/jtreg/pull/77/files/994d0aee..96230a3b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=77&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=77&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/77/head:pull/77

PR: https://git.openjdk.java.net/jtreg/pull/77