Restore stress_test_common script.

Restore stress_test_common script.

This script were present in https://codereview.chromium.org/22935012
but it disappeared when that CL was commited manually.
Also fix copy-paste error in analyze_stress_test.

TEST= courgette/run_stress_test -s <chrome_dir1> <chrome_dir2> <result_dir>
      courgette/run_mem_test <result_dir>
      courgette/analyze_stress_test <result_dir>/log
      courgette/analyze_mem_test <result_dir>
Also create fake bsdiff and bspatch which use courgette executable and
add courgette and these utilities to the PATH.
BUG= 274054
R=dgarrett@chromium.org

Review URL: https://codereview.chromium.org/994883002

Cr-Commit-Position: refs/heads/master@{#320250}