Flip the order of the diff in test-plugin (#2059)

When this failed on CI the other day it looked a little odd as the "-"
lines were the ones that were the new state. So flip the order so the
"+" lines are how it should be generated vs. what is currently checked
in to the Reference directory. i.e. - the output becomes "the patch to
apply to reference" instead of the output being what would get applied
to the "generator".

Author: thomasvl

Makefile

@@ -238,7 +238,7 @@ test-plugin: build ${PROTOC_GEN_SWIFT} ${PROTOC}
 		--tfiws_opt=EnumGeneration=NonExhaustive \
 		--tfiws_out=_test/Tests/protoc-gen-swiftTests/NonExhaustive \
 		Protos/Tests/protoc-gen-swiftTests/enum_generation_test.proto
-	diff -ru _test Reference
+	diff -ru Reference _test
 
 # Test the SPM plugin.
 test-spm-plugin: