selftests/thermal: remove duplicate newlines in perror calls

selftests/thermal: remove duplicate newlines in perror calls

perror() automatically appends a newline character, so the explicit '\n'
in the format strings is redundant and results in duplicate newlines in
the output.

Remove the redundant '\n' characters from perror() calls in
workload_hint_test.c to fix the formatting.

Link: https://lkml.kernel.org/r/F482FB1EC020000C+20250710134751.306096-1-wangyuli@uniontech.com
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Cc: Guan Wentao <guanwentao@uniontech.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>