tests: add lib/spack/spack/test/.cache to gitignore for pytest

- this shoudl have been moved when we moved pytest.ini to lib/spack/spack/test

.gitignore

@@ -21,7 +21,7 @@
 .coverage
 #*
 .#*
-/.cache
+lib/spack/spack/test/.cache
 /bin/spackc
 *.in.log
 *.out.log