Add in a set of .expected files for additional Load checks

Refs #10150
Add some more test files for Load along with .expected files.

Refs #10150
First cut at LoadTest that covers what the unit test was doing.

Refs #10150
Extend LoadTest to cover the functionality that the unit test covered.

There are also extra checks to confirm that the multi-file loading
behaviour is correct.
Refs #10150
Use the correct variable name in the .expected files.

Refs #10150
Fix bug with .expected files

Refs #10150
Merge remote-tracking branch 'origin/maint/10150_improve_loadtest_performance'