gdb: fix field names of GDB's type main_type structure

In commit:

  commit 5b7d941b
  Date:   Fri Jan 22 12:21:09 2021 -0500

      gdb: add owner-related methods to struct type

two fields of struct maint_type were renamed.  'flag_objfile_owned'
became 'm_flag_objfile_owned' and 'owner' became 'm_owner'.  Update
our python helper script to take this into account.

I've added a basic test that uses the self-test framework to load the
pretty printers, and print a type.

The test relies on stopping in GDB's `value_print` function.

gdb/ChangeLog:

	* gdb-gdb.py.in (StructMainTypePrettyPrinter) <owner_to_string>:
	Updated fields names flag_objfile_owned to m_flag_objfile_owned,
	and owner to m_owner.

gdb/testsuite/ChangeLog:

	* gdb.gdb/python-helper.exp: New file.