[rt46602] Ensure test output in systests.output is not mixed up

[rt46602] Ensure test output in systests.output is not mixed up

When running all the system tests, output from a test is sent to a
test.output file in the test directory.  These are combined in to
systests.output when the run finishes.

(cherry picked from commit 055e5be9fd7555e24449157bc78b8c0852f9b7d1)
(cherry picked from commit 50769a4a9e699b74b2a6fa2d5dc91ca54c99d9f3)