jtreg shell tests

[Martin Buchholz]

I agree with Jonathan.

For tests like my ancient MineField.sh it's hard for java code to beat
the compactness of bash code for testing command execution, so I agree
that shell execution  should remain an option, while continuing to
make it easier to write both shell and java tests that spawn
subprocesses.

Some Google perspective: we also have bash libraries.  They might get
source'd multiple times, so should be idempotent (we actually use C
style include guards in bash libraries).

I used to believe that standalone execution of tests was a nice
feature, but agree it's less of a concern now.  It's very nice for
jtreg to provide a -show:rerun feature.  One reason for running the
test outside of the test harness is to insert a wrapper around the
test process, e.g. point LD_LIBRARY_PATH at a debugging malloc or run
under "strace -c" to measure syscalls.  So we have a --run_under=...
flag that takes care of many use cases for standalone execution.