weston-simple-egl: make FPS capture and gating robust

Use run_with_timeout with stdbuf -oL -eL so weston-simple-egl output is line-buffered
and FPS lines reliably land in weston-simple-egl_run.log.

Log weston-simple-egl output into weston-simple-egl_run.log and compute average FPS
across all "N frames in M seconds: X fps" lines instead of relying on a single sample.
Record min, max and average FPS and include them in the result summary for easier
performance debugging.

Gate PASS/FAIL primarily on run duration and optional FPS requirement (REQUIRE_FPS),
using EXPECT_FPS and FPS_TOL_PCT when FPS gating is enabled. Default REQUIRE_FPS=1
to enable FPS checks by default, while keeping behaviour configurable via the
environment.

Keep the .res file format unchanged, still emitting a simple
"weston-simple-egl PASS/FAIL" line so the existing Runner harness continues to work
without changes.

Signed-off-by: Srikanth Muppandam <[email protected]>

Author: smuppand