testsuite: Rename "diff" test to "not-diff"

3 things you need to know about this change:

1. We use diff(1) in various tests to check generated text against
   reference output
2. Windows locates executables not just in $PATH, it also looks in
   $cwd and the directory of the current process' binary
3. Multiple tests live together in the same directory

Windows is fun.

testsuite/gsk/meson.build

@@ -570,8 +570,8 @@ internal_tests = [
   [ 'boundingbox'],
   [ 'curve', [ ], [ 'flaky' ]],
   [ 'curve-special-cases' ],
-  [ 'diff' ],
   [ 'half-float' ],
+  [ 'not-diff' ],
   [ 'misc'],
   [ 'path-private' ],
   [ 'rounded-rect'],