Investigate increased memory usage in tests when calling migrate without available_apps set

[Tim Graham]

91023d79ec70df9289271e63a67675ee51e7dea8 increased memory usage of the test suite. I've created a ​PR to revert it but the root cause should be investigated.

I added these lines at the end of runtests.py to measure memory usage (may be a naive solution):

import resource
print(resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1000)

[Tim Graham <timograham@…>]

In 7d50d2b:

Refs #27790 -- Reverted "Removed available_apps on TestCase subclasses."

This reverts commit 91023d79ec70df9289271e63a67675ee51e7dea8 as it
increases memory usage for the test suite.

[Tim Graham <timograham@…>]

In aa52e8b7:

[1.11.x] Refs #27790 -- Reverted "Removed available_apps on TestCase subclasses."

This reverts commit 91023d79ec70df9289271e63a67675ee51e7dea8 as it
increases memory usage for the test suite.

Backport of 7d50d2bcb8a3cf3fea727174717ead7e6508e9e2 from master