profiler: flush after writing the profiler output
Otherwise, the profiler output might only partially appears until the next flush
of the buffer. Since profiling often happens for long operation, the next flush
can be a long time away.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL