test: Assert that GetValidTransaction supports the given branch ID

Also includes a small documentation fix.

src/gtest/test_checktransaction.cpp

@@ -62,6 +62,9 @@ CMutableTransaction GetValidTransaction(uint32_t consensusBranchId=SPROUT_BRANCH
         mtx.fOverwintered = true;
         mtx.nVersionGroupId = SAPLING_VERSION_GROUP_ID;
         mtx.nVersion = SAPLING_TX_VERSION;
+    } else if (consensusBranchId != SPROUT_BRANCH_ID) {
+        // Unsupported consensus branch ID
+        assert(false);
     }
 
     mtx.vin.resize(2);

src/gtest/test_validation.cpp

@@ -23,7 +23,7 @@ void ExpectOptionalAmount(CAmount expected, boost::optional<CAmount> actual) {
     }
 }
 
-// Fake a view containing a single coin
+// Fake a view that optionally contains a single coin.
 class ValidationFakeCoinsViewDB : public CCoinsView {
 public:
     boost::optional<std::pair<std::pair<uint256, uint256>, std::pair<CTxOut, int>>> coin;