tests: Make sure PATH is set for 'env -i'

'env -i' clears the complete environment, including PATH. In that
case, on most platforms, when excuting commands without PATH being
set, /usr/bin and /bin are searched, e.g. on Linux:

  $ strace env -i asdf |&  grep asdf
  execve("/usr/bin/env", ["env", "-i", "asdf"], 0x7ffc3e3c0890 /* 27 vars */) = 0
  execve("/bin/asdf", ["asdf"], 0x55be2da090d0 /* 0 vars */) = -1 ENOENT (No such file or directory)
  execve("/usr/bin/asdf", ["asdf"], 0x55be2da090d0 /* 0 vars */) = -1 ENOENT (No such file or directory)
  write(2, "\342\200\230asdf\342\200\231", 10‘asdf’) = 10

Howver, this does not hold on every platform. E.g. on Cygwin, so
such fallback paths are searched:

  $ strace env -i asdf |&  grep asdf
     37   25736 [main] env 3516 build_argv: argv[2] = 'asdf'
    643   30373 [main] env 3516 find_exec: find_exec (asdf)
     35   30408 [main] env 3516 find_exec: (null) = find_exec (asdf)
     36   30444 [main] env 3516 spawnve: spawnve (, asdf, 0x10040B000)
  ‘asdf’  199   53601 [main] env 3516 write: 10 = write(2, 0x10040B040, 10)

  $ env -i zsh
  env: ‘zsh’: No such file or directory

Therefore, PATH should be kept as-is when executing the test suite
so that (1) the initial test-highlighting.zsh can be run, and (2)
executables required by the test suite (e.g. tap-colorizer.zsh)
can be executed correctly.

Makefile

@@ -41,7 +41,7 @@ test:
 	for test in highlighters/*; do \
 		if [ -d $$test/test-data ]; then \
 			echo "Running test $${test##*/}"; \
-			env -i QUIET=$$QUIET $${TERM:+"TERM=$$TERM"} $(ZSH) -f tests/test-highlighting.zsh "$${test##*/}"; \
+			env -i QUIET=$$QUIET $${TERM:+"TERM=$$TERM"} PATH="$$PATH" $(ZSH) -f tests/test-highlighting.zsh "$${test##*/}"; \
 			: $$(( result |= $$? )); \
 		fi \
 	done; \