perf tests top: Make the test exclusive

perf tests top: Make the test exclusive

With sufficient tests running the load causes the top test fails with:
```
123: perf top tests                                                  : FAILED!
 --- start ---
test child forked, pid 629856
Basic perf top test
Basic perf top test [Failed: no sample percentage found]
 ---- end(-1) ----
```
Mark the test exclusive to avoid flakes.

Fixes: 75e961730b9e ("perf tests top: Add basic perf top coverage test")
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>