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.

HACKING.md

@@ -6,7 +6,7 @@ core driver (`zsh-syntax-highlighting.zsh`), on the highlighters in the
 distribution, and on the test suite.  It does not target third-party
 highlighter authors (although they may find it an interesting read).
 
-The 'main' highlighter
+The `main` highlighter
 ----------------------
 
 The following function `pz` is useful when working on the `main` highlighting:
@@ -25,6 +25,25 @@ The following function `pz` is useful when working on the `main` highlighting:
 It prints, for each argument, its token breakdown, similar to how the main
 loop of the `main` highlighter sees it.
 
+Testing the `brackets` highlighter
+----------------------------------
+
+Since the test harness empties `ZSH_HIGHLIGHT_STYLES` and the `brackets`
+highlighter interrogates `ZSH_HIGHLIGHT_STYLES` to determine how to highlight,
+tests must set the `bracket-level-#` keys themselves.  For example:
+
+    ZSH_HIGHLIGHT_STYLES[bracket-level-1]=
+    ZSH_HIGHLIGHT_STYLES[bracket-level-2]=
+
+    BUFFER='echo ({x})'
+
+    expected_region_highlight=(
+      "6  6  bracket-level-1" # (
+      "7  7  bracket-level-2" # {
+      "9  9  bracket-level-2" # }
+      "10 10 bracket-level-1" # )
+    )
+
 IRC channel
 -----------