Add test harness for fish command invocation, and tests for init command.

The new '-C' initial command needs some tests, and as there are no
tests just yet for the command invocation, this change adds a harness
and calls it from the high-level tests in the Makefile.

The tests are similar in style to the other high level tests, in that
we capture the output and compare it to that which we expect. The
harness itself is written in bash - sorry - because we're testing the
fish shell's invocation, and trying to do that with the fish we've
just built wouldn't actually make for a very useful test when things
go wrong.

The 'tests/invocation.sh' script can be executed manually, or as part
of the 'make test' target, to make it easy to use both as part of the
development and as part of automation.

The harness has only been tested on linux with bash 4.3.11, and requires
grep and sed. Although not tested with OS X, I believe I have avoided
the syntax which is inconsistent.

The tests added here cover just the initial command's basic execution,
and when it is mixed with the regular '-c' command.

tests/invocation/init-command-mix.invoke

@@ -0,0 +1 @@
+-C 'echo init-command' -c 'echo command'