* Fix a number of misc issues to allow google usage of bazel-skylib
1. Missing copyright header
2. Shell test fixes to use TEST_TMPDIR to have write access to directories
3. diff_test fix to use TEST_SRCDIR
* added a comment as to why diff_test_tests is local
* ran buildifier
This new test rule compares two files and passes
if the files match.
On Linux/macOS/non-Windows, the test compares
files using 'diff'.
On Windows, the test compares files using
'fc.exe'. This utility is available on all Windows
versions I tried (Windows 2008 Server, Windows
2016 Datacenter Core).
See https://github.com/bazelbuild/bazel/issues/5508
See https://github.com/bazelbuild/bazel/issues/4319
