Correct test constraints and add sanity check.

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