Fix sub_test so it only writes the current execution output to the log when running multiple trial for perf tests.

This updates sub_test to read the execution output log after running any
preexecs. The current contents is saved in memory, and re-written to the log
during each execution. Each execution for a performance tests with multiple
trials (--numtrials > 1) needs to truncate the previous execution log, write
the pre-exec output, and then write the current execution output.

Tested against full suite.

paired w/ eronaghan

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23589 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Update file share Nightly results are stored in.

/data/sea/cascade/chapel -> /data/sea/chapel.

r: hildeth

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23590 3a8e244f-b0f2-452b-bcba-4c88e055c3ca