Refactor test-log.cc.

This patch splits the log file into a vector of std::strings when
logging is stopped, so verifying that lines are present can be done in
terms of std library functions. Verifications are now done by simple
substring matching instead of via a prefix or suffix, in preparation for
a new test that needs to match the middle of a line.

This patch also deletes some dead/debugging code.

Change-Id: I5c6b75b0807c41312d35208deda26546dc0f7216
Reviewed-on: https://chromium-review.googlesource.com/1183187Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55281}