feat: Add an executable_test rule and use it in write_source_file

[calliecameron]

Adds executable_test, which checks that a file or directory's executable permissions are as intended. Implementation is mostly copied from diff_test. Tested on Linux, since that's what I have. As far as I remember, the executable permission doesn't exist on Windows, so I've made the Windows version of the test always pass.

Added this to write_source_files, so that when diff_test is true, the executable permission will be checked as well as diffing the contents.

I considered having a separate executable_test arg instead of controlling them both with diff_test - can switch to that if you prefer.

Fixes #1089.

[calliecameron]

@alexeagle any update on reviewing this?

[arrdem]

I see the utility of this functionality but framing it as an "executable" test seems excessively narrow. Given that we use mtree(8) manifests for tar.bzl and rules_oci I wonder if what we really want here is an mtree_test rule, which is already a class of test we have for covering container image contents.

[1] https://man.freebsd.org/cgi/man.cgi?mtree(8)