fix(db test): resolve absolute paths before computing relative path

[7ttp]

when running supabase db test from a docker-outside-of-docker environment, test file paths may be absolute host paths while utils.DbTestsDir is relative. filepath.Rel fails when the base path is relative and the target is absolute, producing the error: Rel: can't make /path/to/supabase/tests relative to supabase/tests. this change converts both paths to absolute before computing the relative path, which handles all path combinations correctly.

cc @sweatybridge

closes https://github.com/supabase/cli/issues/4523