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: 75e9617 ("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>

Author: captain5050

tools/perf/tests/shell/top.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# perf top tests
+# perf top tests (exclusive)
 # SPDX-License-Identifier: GPL-2.0
 
 set -e