Define G_TEST_{SRCDIR,BUILDDIR} during tests
Define G_TEST_{SRCDIR,BUILDDIR} during tests

These environment variables enable finding test fixtures with
g_test_build_filename() regardless of where you are running your tests
from.

In Javascript, we can't use g_test_build_filename() directly, but it's a
simple matter to locate our test fixtures using these environment
variables.

[endlessm/eos-sdk#2635]

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib