test: avoid of too long running sql-tap tests

The test sql-tap/gh-3083-ephemeral-unref-tuples.test.lua took
too long runtime and failed to finish in timeout limit of 2
minutes when it runs near the end of the running queue:

No output during 120 seconds. Will abort after 120 seconds without output. List of workers not reporting the status:
- 012_sql-tap [sql-tap/gh-3083-ephemeral-unref-tuples.test.lua, vinyl] at var/012_sql-tap/gh-3083-ephemeral-unref-tuples.result:0
Test hung! Result content mismatch:
[File does not exists: sql-tap/gh-3083-ephemeral-unref-tuples.result]
[Main process] No output from workers. It seems that we hang. Send SIGKILL to workers; exiting...

Due to suggestion from Konstantin Osipov at gh-3845:
https://github.com/tarantool/tarantool/issues/3845#issue-385735959

The following tests:
  gh-3083-ephemeral-unref-tuples.test.lua
  gh-3332-tuple-format-leak.test.lua
where set to run only on memtx configuration.

Also the test:
  gh-3083-ephemeral-unref-tuples.test.lua
was removed from the test-run 'fragile' option flaky list.

Closed #4128