tests: use NO_CHECK_EOF as heredoc limit mark to omit checking code fragments

This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.

"primes.py" embedded in test-highlight.t causes an error of
check-commit.py below:

    don't use .next(), use next(...)

But changing embedded primes.py is painful, because it is committed in
test script, and affects hash IDs. On the other hand, primes.py itself
is never executed in test script.

Therefore, this patch uses NO_CHECK_EOF as heredoc limit mark in order
to omit any checking on this code fragments.

tests/test-highlight.t

@@ -19,7 +19,7 b''
 
 create random Python file to exercise Pygments
 
-  $ cat <<EOF > primes.py
+  $ cat <<NO_CHECK_EOF > primes.py
   > """Fun with generators. Corresponding Haskell implementation:
   > 
   > primes = 2 : sieve [3, 5..]
@@ -51,7 +51,7 b' create random Python file to exercise Py'
   >         n = 10
   >     p = primes()
   >     print("The first %d primes: %s" % (n, list(itertools.islice(p, n))))
-  > EOF
+  > NO_CHECK_EOF
   $ echo >> primes.py  # to test html markup with an empty line just before EOF
   $ hg ci -Ama
   adding primes.py