Fix gtest test compilation and build more tests

My previous work to integrate these tests was incomplete/incorrect, because I
misunderstood how the cmake macros worked.

This addresses items with the gtest tests, which in turn fixes test compilation
and adds more tests which I had previously missed.

Due to an unknown issue with gtest_stress_test, I had to add pthread to LIBADD,
even though I shouldn't have added it to that (it was failing to link -lpthread
to libprivategtest.a). Add a XXX comment to note that something's awry there
and deserves additional investigation.