[1.6.x] Made Oracle introspect FloatFields correctly

Broke InspectDBTestCase.test_field_types in two:
- a test_number_field_types, which now passes on Oracle too
- a test_field_types, for all non-numeric fields, which is still expected to fail

Also made some pep8 fixes in the tests file. Refs #19884

Thanks Tim Graham for review.

Backport of e9d12bae from master