tests: add a test runner utility that prints nothing when all tests pass
This will be used to run tests through run-tests, which will expect no output
for a unit test that passes successfully.
The motivation for using unit tests instead of the current Python tests is that
they don't require an output file for comparison and that they're easier to write
because of the available tools from unittest (setup, asserts).
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..