It's really important for `git-check-assertions` blocks to be 'obviously
correct', so that we don't need to... test the tests that test the tests.
Let's introduce a little DSL that is less error-prone than a plain bash
script.
```git-check-assertions
run test/git-check-assertions.bats
assert_success
git checkout HEAD~1 bin
run test/git-check-assertions.bats
assert_failure
assert_output --partial "file exists, but it was expected to be absent"
```
```git-check-assertions
sed -i 's/HEAD~/HEAD/' test/git-check-assertions.bats
run test/git-check-assertions.bats
assert_failure
assert_output --partial "Expected command to fail, but it succeeded."
```
