No test because _zsh_highlight_main__type() falls back to 'type -w' which runs
'rehash' implicitly, so on systems where 'sudo' is installed it's not possible
to simulate its being absent.

Test by forcing _zsh_highlight_main__type() to return 'none' when the
argument is [[ $1 == 'sudo' ]], and: (1) Run 'make test' and confirm
that all tests either pass, or fail and the first test point that fails
is one that expects "sudo" at command position to be highlighted as
'command'; (2) In an interactive zsh, 'sudo' at command position is
highlighted as an error.

Closes #458.

Closes #112. Closes #138.

Unbreak build for 5.0.8 and earlier.

Closes #382.

Fixes #449.

The functions can remain defined because, nowadays, the test harness
runs each test in a subshell; but that's a well-known property of the
test harness so need not be mentioned explicitly.

Inspired by discussion on issue #443.

Useful when using, e.g., Debian's package, which outputs just "Debian" here.

Context: zsh-users/zsh-docker#2