Strengthen coverage check for prop_spendTx_balance_inequality.

What we really want here is to capture the notion of "strictly less
than", but `leq` gives us "less than OR equal". We can't use `<=`, as
token maps are only partially ordered. So we have to use a combination
of `leq` and `/=`.

In response to review comment:

https://github.com/input-output-hk/cardano-wallet/pull/2867/files#r701595441

lib/core/test/unit/Cardano/Wallet/Primitive/ModelSpec.hs

@@ -1490,8 +1490,8 @@ prop_spendTx_balance_inequality :: Tx -> UTxO -> Property
 prop_spendTx_balance_inequality tx u =
     checkCoverage $
     cover 10
-        (lhs /= mempty && lhs `leq` rhs)
-        "lhs /= mempty && lhs `leq` rhs" $
+        (lhs /= mempty && lhs `leq` rhs && lhs /= rhs)
+        "lhs /= mempty && lhs `leq` rhs && lhs /= rhs" $
     isJust (rhs `TokenBundle.subtract` lhs)
         & counterexample ("balance (spendTx tx u) = " <> show lhs)
         & counterexample ("balance u = " <> show rhs)