rename custom easyblock parameters

[jfgrimm]

We should go through all the easyblocks to check what custom parameters they define, and ensure those also adhere to our naming policy (see easybuilders/easybuild-framework#4563)

[Flamefire]

Experience from trying to change testinstall to install_for_test in PythonPackage used by PythonBundle:

• PythonBundle tries to set self.cfg['exts_default_options'][key] = self.cfg[key] with key = install_for_test
• The decorator on __get_item__ changes the key to testinstall
• This doesn't exist in PythonBundle (inheriting vars from PythonPackage)
• --> Error "Use of unknown easyconfig parameter 'testinstall' when getting parameter value"