[Toybox] test.sh: Don't override "C" command path in TEST_HOST if it's set

[Oliver Webb]

> On 3/21/24 21:38, Oliver Webb via Toybox wrote:
> 
> > A mildly annoying issue of you are trying to test with different implementations of commands
> > such as plan9 ones or sbase or busybox ones, things with different conflicting implementations
> > of things like xxd or vi. With this patch you can do "make test_cmd TEST_HOST=1 C=/path/to/other/cmd"
> > and have it work
> 
> I've been doing "PATH=/path/to/thingy:$PATH TEST_HOST=1 make test_cmd" for
> years, I didn't know that needed to be documented...

plan9 has a incompatible diff implementation, which means to test plan9 utils I'd
either need to separate diff from the rest of the binaries or have some way of overriding "C".

Also, different command names, there's a dozen different vi implementations and 
only a few have the name "vi". This is true for some other commands as well