chore: report file location of the integration test that failed

[alecthomas]

By adding a bunch of t.Helper() calls.

Before:

=== NAME  TestLeaseFailure/go
    /Users/alec/dev/ftl/internal/integration/harness.go:267: Timed out waiting for assertion to pass: Expected values to be equal:
        -error
        +executing

After:

=== NAME  TestLeaseFailure/go
    /Users/alec/dev/ftl/backend/controller/pubsub/integration_test.go:178: Timed out waiting for assertion to pass: Expected values to be equal:
        -error
        +executing
--- FAIL: TestLeaseFailure (22.26s)
    --- FAIL: TestLeaseFailure/go (18.18s)