MTR --mysqld=--default-storage-engine=foo incompatible w/ dynamically-loaded foo

Copy of http://bugs.mysql.com/bug.php?id=72615:

[12 May 8:29] Laurynas Biveinis
Description:
Credit goes to Rich Prohaska, https://github.com/Tokutek/percona-server-5.6/issues/8

MTR checks for the presence of default-storage-engine and default-tmp-storage-engine mysqld options (command_line_setup, optimize_cases) and then tests whether the specified engine is actually supported by the server (optimize_cases).

The test for support is performed by using server variables saved from an initial mysqld --help run (collect_mysqld_features). mysqld --help does not add any --plugin-load options there. Now, if the storage engine is dynamically loaded, and MTR is invoked with --mysqld=--default-storage-engine=foo --mysqld=--plugin-load=foo=foo.so, its presence won't be registered by collect_mysqld_features and the testcases will be skipped although they shouldn't.

How to repeat:
See above.

Suggested fix:
Pass any specified plugin-load options to collect_mysqld_features.