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

[Leonid Mesnik]

On Thu, 9 Mar 2023 10:23:53 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

> 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.`

Marked as reviewed by lmesnik (Reviewer).

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