[test] Do not overwrite test file in clang_compilation_db_* tests

Summary:
This diff avoids overwriting test files in the clang_compilation_db_escaped/realpath tests. Both Makefiles of the tests are running below:

```
$(COPY) "../codetoanalyze/path_with_spaces/hel_lo.c" "../codetoanalyze/path with spaces/hel lo.c" \
|| diff "../codetoanalyze/path_with_spaces/hel_lo.c" "../codetoanalyze/path with spaces/hel lo.c"
```

If they run in a bad timing, at the same time:

* one is trying to copy the target file (write)
* one is trying to diff the target file (read)

it can introduce the `diff` running failure. This diff tries to avoid the overwriting at the copy to avoid the timing issue.

Reviewed By: geralt-encore

Differential Revision: D49597444

fbshipit-source-id: d6ef35b9246230dbaa0b32b8a30ee504bf1c4c4e

Makefile.config

@@ -117,7 +117,7 @@ XCODE_ISYSROOT := $(XCODE_BASE)/$(XCODE_ISYSROOT_SUFFIX)
 endif
 
 define copy_or_same_file
-  $(COPY) "$(1)" "$(2)" || diff -q "$(1)" "$(2)"
+  $(COPY) -n "$(1)" "$(2)" || diff "$(1)" "$(2)"
 endef
 
 INTERACTIVE := $(shell [ -t 0 ] && ! [ x"$$TERM" = xdumb ] && echo 1)