These names can be any mesh generator in the input file, they're not
mesh filenames.

If we don't assume that the index names and the mesh ptr vectors are the
same size, then we can have subclasses that add additional mesh ptr
entries from other sources.  This is useful for a test subclass I'm
adding.

The modified test will instantiate "subgenerators" itself instead of
getting them from earlier in the input file.

Template parameter packs are neat.

More importantly, template parameter packs let us pass arbitrary
parameters to set in the internals of another helper function.

Hopefully this is something like what the ANL folks had in mind for
idaholab#17184

This turns several lines of slightly tricky code into 4 lines of
arguably self-documenting code, so I'm fairly happy with it.

Make subgenerator name base unique

Co-authored-by: Logan Harbour <loganhharbour@gmail.com>

Thanks to @loganharbour for catching this

This overload is a little trickier to use but is more in line with some
other MOOSE APIs.

This lets users be a little less picky about how they construct their
InputParameters without us hitting a failure at generate() time.

This way, if we ever add any more subtle parameters like _moose_app that
ought to be set from within addMeshSubgenerator() itself, we'll catch
the error in this test rather than in any user codes making use of the
InputParameters-accepting overload.

This is a test object only

Co-authored-by: Logan Harbour <loganhharbour@gmail.com>

Or a requirement that @loganharbour and I were both happy enough with,
at least.

Logan is insistant that test objects should be clearly labeled as such.

Which makes sense, so I nominate him to fix the *other* 291 classes in
test/src/*/ that don't have "Test" in the name.