Audit how `t.Skip()` is being used in tests

It was discovered in dolthub/go-mysql-server#3607 that the majority of tests in `TestQueryPlans` (test cases in `PlanTests`) were getting skipped because `t.Skip` was being called in the incorrect run. This has since been fixed, but we need to make sure that in our other tests, `t.Skip()` is actually being called in the correct place to avoid skipping tests that aren't supposed to be skipped.

Start by searching the Go test files for t.Skip() and inspect the TestQueryPlans cases in PlanTests, using the issue that prompted this audit as context. Check that each skip is reached only in the intended test run, and verify that tests are no longer skipped incorrectly.

Written by the indexing model from the issue text.