Flakes: Shutdown vttablet before mysqld in backup tests

[mattlord]

Description

In general, it's best to shutdown a tablet before its mysqld instance. This ensures that the tablet can perform any shutdown related steps that may affect its mysqld instance.

The existing backup tests were NOT doing that, and the re-use of the MySQL instances could cause some unpredictable behavior.

This PR simply reverses the order of process termination in these tests.

Note: this is a "temporary" fix as these tests need to be rewritten soon as they rely on long deprecated things that will be removed such as InitTablet and InitShardPrimary (not to mention the odd sharing of processes and state between tests).

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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