Minitest will now exit 1 when a `-n` type test filter is used and no
test cases end up being run. However, these tests are not really
concerned with the exit code, just that `test:prepare` doesn't run.

This commit adds `allow_failure: true` to the test command invocations
so that we can continue to test whether `test:prepare` is run without
caring about the exit status of the test command.