elements/base/nss.bst: disable gtest

gtest (googletest) may be used by nss to do some internal testing during build. It's not useful otherwise and exporting it to runtime was a unnecessary. Moreover it can break ABI which therefore is blocking us from updating nss in fd.o.

@bochecha [Freenode IRC Network]
right, so nss has a testsuite using googletest (aka gtest)
see the nss/gtests folder
in it, there are tests… and the google_test source itself, bundled
when we build nss, we also build the tests, and as such the google_test harness
which means we build libgtest.so
and then we install it
we should not install it at all
(and ideally, we also wouldn't build it at all, but that's another issue)`

This is part of moving !825 (merged) forward.

This change will intentionally break ABI (libgtest1.so and libgtestutil.so will be removed from image) so perhaps it has to be merged by @jjardon directly.