Add top-level Makefile target for running gtest

[Jesper Wilhelmsson]

Looks good!
/Jesper

Den 17/5/16 kl. 10:16, skrev Erik Helin:
> Hi all,
>
> this small patch adds a top-level Makefile target for conveniently
> running the gtest unit tests.
>
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8157028
>
> Webrev:
> http://cr.openjdk.java.net/~ehelin/8157028/webrev.00/
>
> Testing:
> - Running locally on Linux x86-64
>
> Thanks,
> Erik
>