Avoid spurious failures of MySQL INSERT packet overflow test

Avoid spurious failures of MySQL INSERT packet overflow test

This test creates a MySQL table called 'test'. In several cases, I have seen a spurious
test failure (in CI) with an error message saying: "table 'test' already exists".

It may be that another test had used a table with the same name and not cleaned it out
correctly. Or maybe we have multiple tests running in parallel in some CI environments,
or the same test DB being used for multiple runs of the test suite.

In any case, change the table name so it is exclusive to this test case only. Also, if
the test table exists at the beginning of the test, drop it.

Closes GH-5479