tests: enforce use of $PYTHON in tests

tests/test-check-shbang.t

@@ -5,9 +5,20 b''
 
 look for python scripts that do not use /usr/bin/env
 
-  $ hg files 'set:grep(r"^#!.*?python") and not grep(r"^#!/usr/bi{1}n/env python")'
+  $ hg files 'set:grep(r"^#!.*?python") and not grep(r"^#!/usr/bi{1}n/env python") - **/*.t'
   [1]
 
+In tests, enforce $PYTHON and *not* /usr/bin/env python or similar:
+  $ hg files 'set:grep(r"#!.*?python") and **/*.t' \
+  > -X tests/test-check-execute.t \
+  > -X tests/test-check-module-imports.t \
+  > -X tests/test-check-pyflakes.t \
+  > -X tests/test-check-shbang.t
+  [1]
+
+The above exclusions are because they're looking for files that
+contain Python but don't end in .py - please avoid adding more.
+
 look for shell scripts that do not use /bin/sh
 
   $ hg files 'set:grep(r"^#!.*/bi{1}n/sh") and not grep(r"^#!/bi{1}n/sh")'