glcpp-test: Capture the stderr output of the preprocessor.

This allows writing tests that verify diagnostics from the preprocessor.

src/glsl/glcpp/tests/glcpp-test

@@ -9,7 +9,7 @@ clean=0
 echo "====== Testing for correctness ======"
 for test in *.c; do
     echo -n "Testing $test..."
-    ../glcpp < $test > $test.out
+    ../glcpp < $test > $test.out 2>&1
     total=$((total+1))
     if cmp $test.expected $test.out >/dev/null 2>&1; then
 	echo "PASS"