Add missing newline in util_ChainMerge test

This was causing a lot of test cases not not be very meaningful because
multiple configuration options were combined into one line.

The changes in test output with this fix make sense and look like:

```diff
- testnet=1 regtest=1 || test
+ testnet=1 regtest=1 || error: Invalid combination of -regtest, -testnet and -chain. Can use at most one.
```

Issue was reported and debugged by
Wladimir J. van der Laan <[email protected]> in
bitcoin#17385 (comment)

src/test/util_tests.cpp

@@ -890,6 +890,7 @@ BOOST_FIXTURE_TEST_CASE(util_ChainMerge, ChainMergeTestingSetup)
             desc += " ";
             desc += argstr + 1;
             conf += argstr + 1;
+            conf += "\n";
         }
         std::istringstream conf_stream(conf);
         BOOST_CHECK(parser.ReadConfigStream(conf_stream, "filepath", error));
@@ -928,7 +929,7 @@ BOOST_FIXTURE_TEST_CASE(util_ChainMerge, ChainMergeTestingSetup)
     // Results file is formatted like:
     //
     //   <input> || <output>
-    BOOST_CHECK_EQUAL(out_sha_hex, "94b4ad55c8ac639a56b93e36f7e32e4c611fd7d7dd7b2be6a71707b1eadcaec7");
+    BOOST_CHECK_EQUAL(out_sha_hex, "f0b3a3c29869edc765d579c928f7f1690a71fbb673b49ccf39cbc4de18156a0d");
 }
 
 BOOST_AUTO_TEST_CASE(util_FormatMoney)