lisa.tests.base: Create the TraceView in RTATestBundle.trace instead of get_trace()

This allows trace_window() to call get_trace() without fearing infinite
recursion. Since all test code is using the "trace" property, the fact that
RTATestBundle.get_trace() will not return the view fitting the rtapp tasks is
not a real issue. Also, get_trace() keeps a semantic of being a "bare" trace,
without extra class-specific filtering on top.