Turned coverage off by default, and fixed missing directory

I turned SimpleCov coverage off by default since the setup takes a little
extra effort. Hopefully this can be streamlined with a script in the future
so anyone can see the coverage that the unit tests provide. The tests now run
with ./tc_all will run without coverage, while './tc_all --coverage' will
attempt to load SimpleCov (see the environment check at the top of ./tc_all).

When setting this project up again, I realized we were missing the test
directory that the unit tests use to manipulate the test harness. I put
a tiny empty (hidden) file so the test directory will be kept in the
repository; I don't think you can commit an empty directory.

modified:   tc_all
modified:   tc_all.rb
new file:   test/.empty_file

Testing:
	o Ran ./tc_all, observed all tests pass