logging: improve the AsyncFileWriter flush test()

logging: improve the AsyncFileWriter flush test()

Summary:
This test has run into occasional failures on continuous build test runs.
Unfortunately when something goes wrong it crashes in the std::thread
destructor due to this thread still being joinable when it is destroyed, which
hides information about what actually failed in the test.

This updates the test to immediately detach the thread, so that on error we
will be able see the real failure reason.

This also increases the size of the message that we write, which will hopefully
help ensure that this write always blocks.

Reviewed By: wez

Differential Revision: D5295574

fbshipit-source-id: ea8cfa855613398f88f9f982c600ec661018a31c