tests/sys/pmc: Keep atf_fail reasons on a single line

[ojanerif_amd.com]

Summary

Several tests put the output of pmc record, pmc info or pmcstat into the
atf_fail reason. That output spans several lines, and ATF reports a
multi-line reason as a broken test instead of a failed one, with the
captured output mangled into the error.

Copy the output to stderr, where kyua keeps it in the report, and use a
single-line reason.

Fixes: 546a76463123
Sponsored by: Advanced Micro Devices, Inc.
Signed-off-by: Osvaldo Janeri Filho <[e-mail]>

Test Plan

Ran the tests against a pmc(8) without the fix in the parent review, so
that pmc record fails. pmc_record_then_info and pmc_frontend_ibs were
reported as "broken: Test result contains multiple lines" before this
change and as failed with a one-line reason after it, with the pmc
record output in the test's stderr. On the same host pmcstat -S fails,
which exercises the pmcstat_tsc_test path: its four cases report
"failed: pmcstat failed" with the pmcstat error in stderr.

[mhorne]

LGTM!