Fix instructions for running tests on Windows

In the command `nmake TEST='foo' test`, on Windows the runner
will look for test `'foo'` and complain about the test not being found
(due to the extraneous single quotes), whereas with `nmake TEST="foo" test`,
the test `foo` will be correctly found.

CLA: trivial

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #23059)

test/README.md

@@ -42,7 +42,7 @@ the make variable TESTS to specify them, like this:
 
     $ make TESTS='test_rsa test_dsa' test            # Unix
     $ mms/macro="TESTS=test_rsa test_dsa" test       ! OpenVMS
-    $ nmake TESTS='test_rsa test_dsa' test           # Windows
+    $ nmake TESTS="test_rsa test_dsa" test           # Windows
 
 And of course, you can combine (Unix examples shown):