tests: Set ZSH_HIGHLIGHT_STYLES=() during tests

Since the _zsh_highlight_add_highlight that the tests use ignores
ZSH_HIGHLIGHT_STYLES, we can test both an injective mapping and an empty
mapping at once.

highlighters/brackets/test-data/mismatch-patentheses.zsh

@@ -27,6 +27,9 @@
 # vim: ft=zsh sw=2 ts=2 et
 # -------------------------------------------------------------------------------------------------
 
+ZSH_HIGHLIGHT_STYLES[bracket-level-1]=
+ZSH_HIGHLIGHT_STYLES[bracket-level-2]=
+
 BUFFER='echo ({x}]'
 
 expected_region_highlight=(