tests: Set ZSH explicitly make sure PATH is exported

'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, we need to make sure that the default PATH is exported
from tests/test-highlighting.zsh.

Makefile

@@ -4,7 +4,7 @@ INSTALL?=install -c
 PREFIX?=/usr/local
 SHARE_DIR?=$(DESTDIR)$(PREFIX)/share/$(NAME)
 DOC_DIR?=$(DESTDIR)$(PREFIX)/share/doc/$(NAME)
-ZSH?=zsh # zsh binary to run tests with
+ZSH?="`which zsh`" # zsh binary to run tests with
 
 all:
 	cd docs && \

tests/test-highlighting.zsh

@@ -29,6 +29,7 @@
 # -------------------------------------------------------------------------------------------------
 
 
+export PATH
 setopt NO_UNSET WARN_CREATE_GLOBAL
 
 # Required for add-zle-hook-widget.