This change adds support for scanning multiple entries in an action string in
order to better support the following use cases:

1. A file is provided in an action string and should be taken as a dependency.
   For example, an action string "$PERL somefile.pl".
2. An action string actually has two actions separated by &&. For example,
   "cd <some_dir> && $ZIP <args>".

Adding support for #1 actually allows us to fix the test
IMPLICIT_COMMAND_DEPENDENCIES.py on Windows, which was previously treating a
Python file as executable even on Windows. This was causing tests to repeatedly
open the default handler of Python files, which if set to Visual Studio causes
DDE hangs. This test is fixed because now we can have the action string specify
python as the first command and still take an implicit dependency on the
script, which is now the second command.

The way runtest.py passes the list of fixture directories is racy because it
sets it in os.environ['FIXTURE_DIRS'] and then spawns the subprocess, counting
on Python to start the subprocess before that list is overwritten when spawning
the next directory. At least on Windows, the environment is not copied in
subprocess.run so runtest.py may overwrite the list of fixture directories
with the list for test #2 while the subprocess module is still kicking off
test #1. I was able to easily reproduce this by running the command:
`python runtest.py -j 2 test\MSVC\VSWHERE.py test\AS\ASPPFLAGS.py`
a few times in a row. However, with this fix, that command repeatedly succeeds.

To validate ths fix, I also ran that command with "--xml a.xml" and
"--xml a.xml --nopipefiles" to validate that those other executors worked
correctly.

Slight reorg of setting default

Clean up logic.