libtest: Make it easier to source interactively

When running the tests by hand, I wanted to be able to source
`libtest.sh` from a running shell.  Make this slightly easier by
allowing one to set `SRCDIR`, since `$0` won't work.

tests/libtest.sh

@@ -25,7 +25,9 @@ if test -n "${LIBTEST_SH:-}"; then
 fi
 LIBTEST_SH=1
 
-SRCDIR=$(dirname $0)
+if test -z "${SRCDIR:-}"; then
+    SRCDIR=$(dirname $0)
+fi
 _cleanup_tmpdir () {
     if test -f ${test_tmpdir}/.test; then
         rm ${test_tmpdir} -rf