'main': Parameter expansions may not contain assignments.

In «a="b=c"; $a», the '=' sign in the expansion of $a is not active.
Therefore, prevent the expansion of $a from being considered an
assignment.  Update test expectations accordingly.

highlighters/main/test-data/parameter-value-contains-command-position2.zsh

@@ -34,5 +34,5 @@ BUFFER=$'$y'
 
 expected_region_highlight=(
   # Used to trigger a "BUG" message on stderr - issues #670 and #674
-  '1 2 assign' # $y
+  '1 2 unknown-token' # $y (not an assignment)
 )