Add support for tests

Right now these are very basic and don't appear to form the correct
parse tree for logical comparisons which use tests as well. But it
parses at lease somewaht correctly, so there is something to work
with here.

tatsu_grammar.txt

@@ -125,7 +125,33 @@ variable_accessor_call_parameter_value_only
 
 variable_accessor_call_parameter_value
     =
-    {SP}* @:variable_identifier {SP}*
+    {SP}* @:( variable_tests | variable_identifier ) {SP}*
+    ;
+
+variable_tests
+    =
+    left:variable_identifier {SP}*
+    (
+        | ( "is" {SP}* test_function:variable_accessor_call_parameter_value )
+        | ( comparator:variable_tests_comparator right: variable_accessor_call_parameter_value )
+        | ( logical_operator:variable_tests_logical_operator right: variable_accessor_call_parameter_value )
+    )
+    ;
+
+variable_tests_comparator
+    =
+    | "=="
+    | "!="
+    | ">"
+    | ">="
+    | "<"
+    | "<="
+    ;
+
+variable_tests_logical_operator
+    =
+    | "and"
+    | "or"
     ;
 
 variable_accessor_dot

test_template.jinja

@@ -18,4 +18,15 @@ across lines #}
                          ('downloads.html', 'Downloads')] %}
     <li><a href="{{ href }}">{{ caption }}</a></li>
 {% endfor %}
-{% trans user=user.username %}Hello, {{ user }}!{% endtrans %}
+{% trans user=user.username %}Hello, {{ user }}!{% endtrans %}
+{% set navigation = [('index.html', 'Index'), ('about.html', 'About')] %}
+{% set key, value = call_something() %}
+{% for value in values %}
+    {% if loop.previtem is defined and value > loop.previtem %}
+        The value just increased!
+    {% endif %}
+    {{ value }}
+    {% if loop.nextitem is defined and loop.nextitem > value %}
+        The value will increase even more!
+    {% endif %}
+{% endfor %}