travis/gitlab/azure: Use bash to avoid a_test_which_does_not_exist

Bash allows for variables to expand only if non-empty:

	$ var=test
	$ echo ${var:+"$var"}
	test
	$ echo ${var:+"-k $var"}
	-k test
	$ var=
	$ echo ${var:+"-k $var"}

Use this feature to avoid the workaround.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/.azure-pipelines.yml b/.azure-pipelines.yml
index e1e69c1..8f9580b 100644
--- a/.azure-pipelines.yml
+++ b/.azure-pipelines.yml
@@ -276,7 +276,8 @@
           pip install -r test/py/requirements.txt
           export PATH=/opt/qemu/bin:/tmp/uboot-test-hooks/bin:${PATH};
           export PYTHONPATH=/tmp/uboot-test-hooks/py/travis-ci;
-          ./test/py/test.py --bd ${TEST_PY_BD} ${TEST_PY_ID} -k "${TEST_PY_TEST_SPEC:-not a_test_which_does_not_exist}" --build-dir "$UBOOT_TRAVIS_BUILD_DIR";
+          # "${var:+"-k $var"}" expands to "" if $var is empty, "-k $var" if not
+          ./test/py/test.py --bd ${TEST_PY_BD} ${TEST_PY_ID} ${TEST_PY_TEST_SPEC:+"-k ${TEST_PY_TEST_SPEC}"} --build-dir "$UBOOT_TRAVIS_BUILD_DIR";
           ret=$?;
           if [[ $ret -ne 0 ]]; then
               exit $ret;