Remove tests for specific file types.

These are covered by the more LoadLotsOfFiles system test and offer no
additional benefit.
Refs #10150
Don't execute a load just test what would Loader was chosen.

Refs #10150
Remove a lot of Load executions in LoadTest

Instead just check what files it would be expected to Load. A new system
test, LoadTest.py, will be created to do the more exhaustive checking
that loading the files requires.
Refs #10150
Merge remote-tracking branch 'origin/maint/10150_improve_loadtest_performance'