'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/main-highlighter.zsh

@@ -840,7 +840,7 @@ _zsh_highlight_main_highlighter_highlight_list()
         function)       style=function;;
         command)        style=command;;
         hashed)         style=hashed-command;;
-        none)           if _zsh_highlight_main_highlighter_check_assign; then
+        none)           if (( ! in_param )) && _zsh_highlight_main_highlighter_check_assign; then
                           _zsh_highlight_main_add_region_highlight $start_pos $end_pos assign
                           local i=$(( arg[(i)=] + 1 ))
                           if [[ $arg[i] == '(' ]]; then

highlighters/main/test-data/parameter-value-contains-command-position1.zsh

@@ -34,5 +34,5 @@ BUFFER=$'$foobar'
 
 expected_region_highlight=(
   # Used to highlight the "ba" as 'command' because the 'ls' showed through; issues #670 and #674
-  '1 7 assign' # $foobar
+  '1 7 unknown-token' # $foobar (not an assignment)
 )

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)
 )