fix valgrind warning

For the bigpcre test we generate a line that won't fit in the stack
buffer.  We were using the return value from vsnprintf to size the
write, but this is the non-truncated length.

Measure the string again and ensure that it has a trailing newline
Aggregate log files from multiple test instances

This explains why tracking down the error on travis is so difficult:
we run multiple watchman processes and were clobbering the log file
with the most recent one.

Hopefully we'll see a more complete pictur for the next travis run.