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>