zshrc: remove needless test invocation.

Any value returned by which is necessarily executable since otherwise it
would not be valid as a command name (and zsh says, "For each name, indicate
how it would be interpreted if used as a command name.").  Omit the needless
invocation of test and instead use plain which.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>