transition: Use stbt.frames instead of stbt.get_frame
transition: Use stbt.frames instead of stbt.get_frame

Since v28, `get_frame()` might return the same frame twice in a row.
For details see https://github.com/stb-tester/stb-tester/pull/449

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester