FrameTraceTest: Account for extra perfetto trace packet

Due to a bug in perfetto, the last trace packet was always lost. The
test worked around that by making sure that there always was a
meaningless packet at the end, often created with traceTimestamp(...
UNSPECIFIED).

Now the bug in perfetto is being fixed.

We cannot simply remove traceTimestamp(... UNSPECIFIED) everywhere
because often that also generated useful packets.

This CL fixes the problem by updating the expectations to consider this
last packet as well.

Also, some EXPECT are converted to ASSERT, to prevent accessing invalid
memory if the tests fail.

Bug: 162206162
Change-Id: Ibf7b83bd3953ab575ea6ebc350123cea830fecd2