drop the base constraint from the cabal test target

Presumably we still have some lower bound but I don't know what it is and if
it's below 8.6.5 ... it doesn't matter to anyone that _I_ know of.

tahoe-ssk.cabal

@@ -135,7 +135,7 @@ test-suite tahoe-ssk-test
   build-depends:
     , asn1-encoding
     , asn1-types
-    , base            ^>=4.14.3.0
+    , base
     , base32
     , binary
     , bytestring