Fix instructions for running tests on Windows

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 https://github.com/openssl/openssl/pull/23059)