This is a regression caused by introducing rendered migration states in
1aa3e09c and the _meta refactoring in fb48eb05.

Thanks to Danilo Bargen for reporting the issue and Marten Kenbeek and
Tim Graham for triaging the bug and providing the initial test case.

The test failed on RHEL6 systems running python 2.7 from a RedHat
Software Collection (SCL) because this test runs an external python
script with a stripped system environment. RedHat SCLs work by setting
a number of system environment variables when these are stripped out by
this test the python 2.7 interpreter is no longer able to function
properly because it can not find the system libraries needed.

Now we use use mock to modify the system environment directly.

Thanks to Andrew Ingram for the report.

Thanks to Reto Aebersold for reporting the issue and Tim Graham and
Claude Paroz for the review.

Changed squashmigrations to not instantiate a MigrationExecutor,
but to directly use a MigrationLoader instance instead.

I changed "appears" to "appear" and emphasized the word "not" to match
the rest of the document.

The locale code can contain a variant after @, so allowed that.

Thanks Carl Meyer for review.

Thanks datakid for the report

Refs #15053.

This now works for multiple Django engines and recursive loaders.
Support for non-Django engines is still pending.

Refs #15053.