test: Rename handle-events.c to common.c

Previously, this C file defined one function
(handle_events). Recently, it acquired a second function
(create_context).

In order to provide a common namespace, these functions are here
renamed to:

	common_handle_events
and
	common_create_context

so the filename is renamed to match.

Similarly, the HANDLE_EVENTS_GL_PREFIX macro is also renamed to
COMMON_GL_PREFIX.