perf testsuite: Fix shellcheck warnings

Shellcheck is becoming a standard when building perf to prevent
any unnecessary mistakes. Fix shellcheck warnings in perf testsuite.

Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240702110849.31904-5-vmolnaro@redhat.com


Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>