MTR and server writes can interleave in the error log

mtr writes to the server error log to mark current test.
Server writes to its error log at the same time.

On Windows, these writes can interleave, since appends to the log file are not atomic.
Sometimes, some lines from server get lost. Sometimes some lines from mtr get lost
This confuses MTR.

An example output from
http://buildbot.askmonty.org/buildbot/builders/win32-debug/builds/1117/steps/test/logs/mysqld.1.err.3
showing mtr overwriting server output

CURRENT_TEST: main.flush_block_commit_notembedded
d/sql//Debug/mysqld.exe (root[root] @ localhost [::1]): Normal shutdown