glsl/glcpp: Add testing for null directives with spaces and comments

This new "make check" test stresses out the support from the last two commits,
(to esnure that '#' is correctly interpreted as the null directives,
regardless of any whitespace or comments on the same line).

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/glcpp/tests/140-null-directive.c

@@ -0,0 +1,9 @@
+/* GLSL accepts a null directive. Let's test that in several variations: */
+#
+        #        
+/*....*/#/*....*/
+ /*..*/ # /*..*/ 
+#//...
+        #   //...
+/*....*/#/**///..
+ /*..*/ # /**/ //

src/glsl/glcpp/tests/140-null-directive.c.expected

@@ -0,0 +1,9 @@
+ 
+
+
+
+
+
+
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+