tests: Don't run HTTP digest tests for SSPI based builds

Added !SSPI to the features list of the HTTP digest tests, as SSPI
based builds now use the Windows SSPI messaging API rather than the
internal functions, and we can't control the random numbers that get
used as part of the digest.

tests/data/test2026

@@ -151,6 +151,10 @@ Finally, this is the real page!
 <server>
 http
 </server>
+<features>
+!SSPI
+crypto
+</features>
 <tool>
 libauthretry
 </tool>