Color `diff` output in `tests/functional/lang` tests

Use `diff --color=always` to print colored output for language test
failures. I've also flipped the arguments so that expected lines missing
from the actual output will be marked with a red `-` and additional
lines found in the actual output will be marked with a green `+`.
Previously it was the other way around, which was very confusing.

tests/functional/lang/framework.sh

@@ -16,7 +16,7 @@ function diffAndAcceptInner() {
     fi
 
     # Diff so we get a nice message
-    if ! diff --unified "$got" "$expectedOrEmpty"; then
+    if ! diff --color=always --unified "$expectedOrEmpty" "$got"; then
         echo "FAIL: evaluation result of $testName not as expected"
         badDiff=1
     fi