Eliminate the TestStarted-XXX and TestFinished-XXX files from check-lldb runs.

Nobody seems to know what purpose these files serve, yet they were accumulating by the thousands in the test traces directory.  I'm proposing we delete them.

Creating these files accounted for about 2.5% of the time to run ninja check-lldb on my machine, which isn't a lot, but it's something.

llvm-svn: 263122