Support -skip of multiple tests

> (Not sure if I was doing something wrong or maybeit is just a part of our story that needs more polishing, filling issue to discuss)

i was trying to run tests on CI (https://github.com/ipfs/bifrost-gateway/pull/190) with

```yaml
args: -skip 'TestGatewayCache/GET_for_/ipfs/_with_only-if-cached_succeeds_when_in_local_datastore' -skip 'TestGatewayCache/HEAD_for_/ipfs/_with_only-if-cached_succeeds_when_in_local_datastore'
```

When run, only one of them was excluded.

I worked around it with regex (since bothe differ in HEAD/GET):

```yaml
args: -skip 'TestGatewayCache/.*_for_/ipfs/_with_only-if-cached_succeeds_when_in_local_datastore'
```

but we should make it possible for people running test suite to exclude more than one pattern, as they may want to exclude multiple unrelated ones.