perf test: Show snippet failure output for verbose=1

perf test: Show snippet failure output for verbose=1

Currently, when running tests in verbose mode (-v), if a test case
fails, the entire raw standard error buffer is dumped to stderr via
fprintf(stderr, "%s", child->err_output.buf). For tests that generate
massive amounts of debugging or logging output before dying, this
results in multi-page terminal dumps where highly critical diagnostic
keywords (error, fail, segv) are easily lost.

Implement a smart, bounded snippet string processor to improve
failure triaging:

1. Introduce a configurable quota limit static unsigned int
   failure_snippet_lines = 10; accessible via a new command-line option
   --failure-snippet-lines <N>.

2. Parse the raw error buffer dynamically into lines and run a
   three-pass extraction algorithm:
   - Pass 0: Always select the very first line of the log as an initial
     outline marker.
   - Pass 1: Scan forward from the top of the log to pick up to N lines
     that contain case-insensitive failure keywords (error, fail,
     segv, abort) to isolate the root cause. Automatically pull in
     the immediate subsequent line as highly-prioritized context.
     Allow adjacent matching lines to overlap without dropping context
     by evaluating keywords for all lines (e.g. when "Failed to
     report" is followed by "Error:").
   - Pass 2: If quota remains, scan backward from the absolute tail of
     the log to capture trailing crash or abort context.

3. Output the selected lines in their original chronological order,
   inserting a clear ... separator between non-contiguous line jumps.

4. Wrap matched failure keywords dynamically in bold red
   (PERF_COLOR_RED) to immediately draw the eye to failures.

5. Invoke the smart processor purely when verbose == 1 && ret ==
   TEST_FAIL in both finish_test and finish_tests_parallel, leaving
   raw full-output dumping completely untouched when running highly
   verbose (-vv).

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.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>