cmd/promtool: resolve relative paths in alert test files (#5336)
cmd/promtool: resolve relative paths in alert test files (#5336)

Like `promtool check config <path/to/foo.yaml>`, which resolves relative
paths inside foo.yaml to be relative to `path/to`, this now makes
`promtool test rules <path/to/test.yaml>` do the same thing.

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@gmail.com>
prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,kangwoo/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,Gouthamve/prometheus,alileza/prometheus,kangwoo/prometheus