perf test stat+shadow_stat.sh: Add threshold for rounding errors

perf test stat+shadow_stat.sh: Add threshold for rounding errors

The test was failing in specific scenarios due to imperfection of FP
arithmetics. The `bc` command wasn't correctly rounding the result of
division causing the failure.

Replace the `bc` with `awk` which should work with more decimal places
and add a threshold to catch any possible rounding errors.  The
acceptable rounding error is set to 0.01 when the test passes with a
warning message.

Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Acked-by: Michael Petlan <mpetlan@redhat.com>
Link: https://lore.kernel.org/r/20230919150419.23193-1-vmolnaro@redhat.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>