J2 test docs (#16646)

* moved tests from filters to actual jinja2 tests

also removed some unused declarations and imports

* split tests into their own docs

removed isnan as existing jinja2's 'number' already covers same
added missing docs for several tests

* updated as per feedback

lib/ansible/plugins/test/mathstuff.py

@@ -18,12 +18,20 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+import math
+
 def issubset(a, b):
     return set(a) <= set(b)
 
 def issuperset(a, b):
     return set(a) >= set(b)
 
+def isnotanumber(x):
+    try:
+        return math.isnan(x)
+    except TypeError:
+        return False
+
 class TestModule:
     ''' Ansible math jinja2 tests '''
 
@@ -32,4 +40,5 @@ class TestModule:
             # set theory
             'issubset': issubset,
             'issuperset': issuperset,
+            'isnan': isnotanumber,
         }