tests: Split RFC 2616 date parsing code out and add tests

This makes it testable, and increases its test coverage too 100% of
lines, as measured by `make coverage`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

tests/.gitignore

@@ -21,5 +21,6 @@ test-repo
 test-repo-finder-avahi
 test-repo-finder-config
 test-repo-finder-mount
+test-rfc2616-dates
 test-rollsum-cli
 test-kargs