glcpp: Add test for the #error directive.

This directive is already implemented nicely, but wasn't previously tested.
It will be convenient to use this directive in further tests that rely
on error messages, (such as ensuring that #line correctly sets the line
number in the error message).

src/glsl/glcpp/tests/090-hash-error.c

@@ -0,0 +1 @@
+#error human error

src/glsl/glcpp/tests/090-hash-error.c.expected

@@ -0,0 +1,3 @@
+0:1(2): preprocessor error: #error human error
+
+