intel/compiler: link all tests again gtest, even test_eu_compact"

intel/compiler: link all tests again gtest, even test_eu_compact"

At the moment all the tests but test_eu_compact are actual C++ gtests.
To simplify things, we can move the gtest.la to the common TEST_LIBS.
As we're here, we can rename change the test extension [to .cpp] to
avoid using the confusing dummy.cpp.

Add a nice comment in the makefile for posterity.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>