RFR: JDK-8303822: gtestMain should give more helpful output

[Matthias Baesken]

When the JDK to test is not provided, gtestMain currently prints something like this :
./hotspot/variant-server/libjvm/gtest/gtestLauncher
ERROR: You must specify a JDK to use for running the unit tests.

The output could be improved, might be helpful for occasional users, e.g.
`ERROR: You must specify a JDK (-jdk <image>, --jdk=<image> or -jdk:<image>) to use for running the unit tests.`

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Commit messages:
 - JDK-8303822

Changes: https://git.openjdk.org/jdk/pull/12940/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12940&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303822
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12940.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12940/head:pull/12940

PR: https://git.openjdk.org/jdk/pull/12940