libtest.sh: use G_TEST_SRCDIR, G_TEST_BUILDDIR to find resources

libtest.sh: use G_TEST_SRCDIR, G_TEST_BUILDDIR to find resources

This fixes the bug that in installed-tests that run testlib.sh under
"bash -c" (i.e. the C and JS tests), $(dirname $0) is "." and we can't do
the LD_PRELOAD correctly:

ERROR: ld.so: object './libreaddir-rand.so' from LD_PRELOAD cannot be
preloaded (cannot open shared object file): ignored.

Similarly, those tests can't copy gpghome correctly.

This also removes the confusing situation that SRCDIR in libtest.sh
(which is ${top_srcdir}/tests) does not mean the same thing as SRCDIR
in test-abi.sh (which is just ${top_srcdir}).

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #234
Approved by: cgwalters