[tests] Properly test for multiple headers with same name in probes (#9538)

Updates the `responseHeaders` probe checks to properly test for multiple headers with the same name.

Previously the probes were using `headers.get()` which concats multiple headers into a single string, which results in the test not really checking if there are in fact multiple headers with the same name. Using `headers.raw()` allows us to properly test for that.

A couple of Python tests that were already checking for multiple `set-cookie` headers needed to be updated to match the full value, since the check now properly validates the full string match of each header (before it was basically just doing a `string.includes()` check).

This is a precursor for https://github.com/vercel/vercel/pull/9533.

packages/python/test/fixtures/20-multivalue-header/vercel.json

@@ -17,7 +17,7 @@
     {
       "path": "/cookie_wsgi.py",
       "responseHeaders": {
-        "set-cookie": ["one=first", "two=second"]
+        "set-cookie": ["one=first; Path=/", "two=second; Path=/"]
       }
     }
   ]