Correct test constraints and add sanity check.

test/runner/requirements/constraints.txt

@@ -1,2 +1,3 @@
 coverage >= 4.2
+jinja2 < 2.9 # 2.9 introduces changes which break tests
 pywinrm >= 0.2.1 # 0.1.1 required, but 0.2.1 provides better performance

test/runner/requirements/integration.txt

@@ -1,4 +1,4 @@
-jinja2<2.9
+jinja2
 jmespath
 junit-xml
 ordereddict ; python_version < '2.7'

test/sanity/code-smell/test-constraints.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+constraints=$(
+    grep '.' test/runner/requirements/*.txt \
+        | sed 's/ *;.*$//; s/ #.*$//' \
+        | grep -v '/constraints.txt:' \
+        | grep '[<>=]'
+)
+
+if [ "${constraints}" ]; then
+    echo 'Constraints for test requirements should be in "test/runner/requirements/constraints.txt".'
+    echo 'The following constraints were found outside the "constraints.txt" file:'
+    echo "${constraints}"
+    exit 1
+fi