perf beauty mmap_flags: Use "test -f" instead of "[-f FILE]"

perf beauty mmap_flags: Use "test -f" instead of "[-f FILE]"

"[" is part of the shell builtin test (and a synonym for it),
 not a link to the external command /usr/bin/test.

Using the "test" is simpler because it avoids a lot of "[]".

Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Acked-by: Huacai Chen <chenhuacai@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: loongarch@lists.linux.dev
Cc: loongson-kernel@lists.loongnix.cn
Link: https://lore.kernel.org/r/c50bc0a92dce0ff0fa6504c1a52fb53e2ac007bf.1692962043.git.siyanteng@loongson.cn
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>