Add another admonishment to WritingTests.md

The `crypto_curve25519.c` file and the `test_crypto.c` file both define

`CRYPTO_CURVE25519_PRIVATE`, so they can see this declaration.

### STOP!  Does this test really test?

When writing tests, it's not enough to just generate coverage on all the

lines of the code that you're testing:  It's important to make sure that

the test _really tests_ the code.

For example, here is a _bad_ test for the unlink() function (which is

supposed to remove a file).

    static void

    test_unlink_badly(void *arg)

    {

      (void) arg;

      int r;

      const char *fname = get_fname("tmpfile");

      /* If the file isn't there, unlink returns -1 and sets ENOENT */

      r = unlink(fname);

      tt_int_op(n, OP_EQ, -1);

      tt_int_op(errno, OP_EQ, ENOENT);

      /* If the file DOES exist, unlink returns 0. */

      write_str_to_file(fname, "hello world", 0);

      r = unlink(fnme);

      tt_int_op(r, OP_EQ, 0);

    done:

      tor_free(contents);

    }

This test might get very high coverage on unlink().  So why is it a

bad test? Because it doesn't check that unlink() *actually removes the

named file*!

Remember, the purpose of a test is to succeed if the code does what

it's supposed to do, and fail otherwise.  Try to design your tests so

that they check for the code's intended and documented functionality

as much as possible.

### Mock functions for testing in isolation

Often we want to test that a function works right, but the function to