tests/sys/pmc: Pass pmcstat -n before -P in pmc_callchain_ur_test

[ojanerif_amd.com]

Summary

pmcstat -n sets the sampling rate only for the sampling PMCs that follow
it on the command line. pmc_callchain_ur_test passed it after -P, so
UR_RATE was never applied and the PMC sampled at the default rate from
pmc_pmu_sample_rate_get(): 65536 on AMD, and 2000003 for
inst_retired.any on Intel, about 20 times fewer samples than intended.

Move -n ahead of -P in both pmcstat invocations.

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

Test Plan

On an AMD Zen 4 host, five runs of each order with the test's 4 second
workload, counting the increase of kern.hwpmc.stats.merges:

• -P instructions -n 100000 (old order): 973 to 1641, in line with no -n at all (1095), i.e. the default rate of 65536.
• -n 100000 -P instructions (new order): 645 to 784.
• With a rate of 10000 the new order gives 3926 and the old one 1664.

pmc_callchain_ur_test passed 3 of 3 runs with the change. Not tested on
Intel.