perf test: Refactor parallel poll loop to drain all pipes simultaneously

perf test: Refactor parallel poll loop to drain all pipes simultaneously

When running tests in parallel with verbose output (-v), child processes
write to pipes. If a test produces significant output (e.g. Granite
Rapids metric parsing printing hundreds of lines), it fills the 64KB
pipe buffer and blocks.

Previously, the parent harness (finish_test) only polled the pipe of the
current test waiting to be printed. Other children blocked indefinitely
until the parent reached them, severely sequentializing execution.

Address this by implementing finish_tests_parallel() to poll and drain
output pipes from all running children simultaneously into per-child
buffers, employing safe strbuf_addstr string operations alongside
thorough variable orderings for strict ISO C90 compliance. Reaping
occurs out of order as children finish, while final result printing
remains strictly in order.

This drops parallel verbose execution time for the PMU events suite from
~35 seconds down to ~5.9 seconds.

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>