[test] Let perf runner interleave try executions.

[test] Let perf runner interleave try executions.

This allows running two executables interleaved to reduce
flakiness of tryjobs. The executables must reside in
different out directories.

BUG=chromium:507213
LOG=n
NOTRY=true
TEST=python -m unittest run_perf_test

Review URL: https://codereview.chromium.org/1215273003

Cr-Commit-Position: refs/heads/master@{#29572}