Support `test` command

[lowlighter]

Somewhat related to #100, but probably simpler to implement

Combined with && and || you can achieve basic logic which is already supported by the test runner
https://www.man7.org/linux/man-pages/man1/test.1.html

Examples

test -n $CI && echo "ci" || echo "not ci"
test -f artifact
test $VERSION -ge 1