twister: rename variable p

[wangnuannuan]

The variable p represents a platform according to

zephyr/scripts/pylib/twister/twisterlib.py

Line 3302 in 30de4b5

for p in selected:

But it's reassigned if there are failures.

zephyr/scripts/pylib/twister/twisterlib.py

Line 3448 in 30de4b5

p = ("%s/%s/%s" % (self.outdir, instance.platform.name, instance.testcase.name))

This causes the web page of test results to look cluttered. Go to https://testing.zephyrproject.org/daily_tests/zephyr-v2.4.0-3224-gc77ae15c9967/report/index.html, you will find out the message

/SCRATCH2/arcjenkins2/slaves/cn58-custom-linux-EMSK-V22-EM7D/...