introduced by r70934 that was causing a test failure when run under -O.

Fixed incorrect object passed into format_float_internal(). This was resulting in a conversion being done twice.

fix to Issue1161031, refactoring the test to simplify it in
the process.

Issue 5354: Provide a standardised testing mechanism for doing fresh imports of modules, including the ability to block extension modules in order to test the pure Python fallbacks