Add an expected file for 084-unbalanced-parentheses

The expected file here captures the current behavior of glcpp (which
is to generate an obscure "syntax error, unexpected $end" diagnostic
for this case).

It would certainly be better for glcpp to generate a nicer diagnostic,
(such as "missing closing parenthesis in function-like macro
definition" or so), but the current behavior is at least correct, and
expected. So we can make the test suite more useful by marking the
current behavior as expected.

src/glsl/glcpp/tests/084-unbalanced-parentheses.c.expected

@@ -0,0 +1,2 @@
+0:2(8): preprocessor error: syntax error, unexpected $end
+