RFR: 7903809: Invalid source release N with --enable-preview [v3]

[Christian Stein]

> Please review this change to prevent "invalid source release N with --enable-preview" errors for when an explicit compile task targets a different JDK feature release number then the JDK under test (running the compilation).

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Use `String` comparison to support versions < 9, for example `"1.8"`

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/226/files
  - new: https://git.openjdk.org/jtreg/pull/226/files/bb9bb6e3..c9ab9fbf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=226&range=02
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=226&range=01-02

  Stats: 9 lines in 1 file changed: 0 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/jtreg/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/226/head:pull/226

PR: https://git.openjdk.org/jtreg/pull/226