patch(1): add some basic tests

Summary:

• basic: test application of patches created by diff -u at the beginning/middle/end of file, which have differing amounts of context before and after chunks being added
• limited_ctx: stems from PR 74127 in which a rogue line was getting added when the patch should have been rejected. Similar behavior was reproducible with larger contexts near the beginning/end of a file. See r326084 for details
• file_creation: patch sourced from /dev/null should create the file
• file_nodupe: said patch sourced from /dev/null shouldn't dupe the contents when re-applied (personal vendetta, WIP, see comment)
• file_removal: this follows from nodupe; the reverse of a patch sourced from /dev/null is most naturally deleting the file, as is expected based on GNU patch behavior (WIP)