Regression tests: Replace COPY for 1-3 lines with INSERT to speed up regression tests

Earlier Citus versions only supported `COPY` operations. So when we needed to add regression tests to check for constraints for example, we'd create a data file and run `COPY` (or \stage).

As a result, we have 2-3 regression tests that `COPY` in a few rows. These tests are harder to read, and slower to run. See #133 for an example.

Now that we support multi-value `INSERT`s, we can change these regression tests to `INSERT` values instead of `COPY`.

Review the regression tests that use COPY or \stage for only a few rows, using issue #133 as the example to locate the pattern. Replace those small data loads with multi-value INSERT statements and run the affected regression tests to confirm they still cover the same constraints and complete successfully.

Written by the indexing model from the issue text.