perf test: Skip shebang and SPDX comments in shell test descriptions

perf test: Skip shebang and SPDX comments in shell test descriptions

When extracting shell test descriptions in tests-scripts.c, the parser
skipped the first line assuming it was the shebang (#!/bin/sh) and then
read the first comment line on line 2 as the test description.

However, checkpatch.pl expects shell scripts to declare their SPDX
license identifier on line 2 (# SPDX-License-Identifier: ...). This
caused the test harness to extract the SPDX license string as the test
description.

Refactor shell_test__description to use io__getline, skipping both
shebang and SPDX comment lines. This allows shell tests to include
standard SPDX headers without breaking test suite description
extraction.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>