tests: keep disabled tests defined

Some versions of test-unit will fail if an unspecified test is
attempted via "-n", so we need to define an empty test.

We cannot use "skip", either, as that seems exclusive to
minitest; and we won't use minitest since it has more
incompatible changes than test-unit over the last 8 years.

The memory leak test is gone since we're more versed in the
Ruby C API nowadays, modern GCs + mallocs may be less
predictable about releasing memory back to the OS.

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