perf test: Fixes to variable expansion and stdout for diff test

perf test: Fixes to variable expansion and stdout for diff test

When make_data fails its error message needs to go to stderr rather
than stdout and the stdout value is captured in a variable.  Quote the
$err value so that it is always a valid input for test.  This error is
commonly encountered if no sample data is gathered by the test.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250312001841.1515779-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>