RFR: 8157325: gtest tests are not excluded for minimal builds

[Erik Helin]

Hi all,

this small patch ensures that the Makefiles correctly excludes gtest
unit tests for a minimal build. Right now, if you add a test to e.g.
hotspot/native/gc/g1, the test will be included in a minimal build,
which will fail.

Thanks to Erik Joelsson for helping out with this patch!

Bug:
https://bugs.openjdk.java.net/browse/JDK-8157325

Webrev:
http://cr.openjdk.java.net/~ehelin/8157325/webrev.00/

Testing:
- JPRT (with test in hotspot/native/gc/g1)
- Locally on Linux x86-64, both with and without a test in
  hotspot/native/gc/g1, both minimal and full build

Thanks,
Erik