Make XML comparison in the test work for SQL Server too

Adding a new line to work around the problem with Oracle didn't allow
this test to pass with SQL Server which strips the newline if it is
present.

So don't modify the input, but do strip the trailing newline from output
if it's present in it to accommodate Oracle, but to allow SQL Server to
work too.

tests/common-tests.h

@@ -4355,13 +4355,7 @@ TEST_CASE_METHOD(common_tests, "XML", "[core][xml]")
 
     int id = 1;
     xml_type xml;
-
-    // The extra new line is a special hack for Oracle: its getCLOBVal()
-    // seems to reformat the returned XML and, in particular, appends a new
-    // line to it if its last line doesn't contain one already. So if we didn't
-    // append it here the check for round trip below would fail because of the
-    // extra new line.
-    xml.value = make_long_xml_string() + "\n";
+    xml.value = make_long_xml_string();
 
     sql << "insert into soci_test (id, x) values (:1, "
         << tc_.to_xml(":2")
@@ -4375,6 +4369,15 @@ TEST_CASE_METHOD(common_tests, "XML", "[core][xml]")
         << " from soci_test where id = :1",
         into(xml2), use(id);
 
+    // The returned value doesn't need to be identical to the original one as
+    // string, only structurally equal as XML. In particular, extra whitespace
+    // can be added and this does happen with Oracle, for example, which adds
+    // an extra new line, so remove it if it's present.
+    if (!xml2.value.empty() && *xml2.value.rbegin() == '\n')
+    {
+        xml2.value.resize(xml2.value.length() - 1);
+    }
+
     CHECK(xml.value == xml2.value);
 
     sql << "update soci_test set x = null where id = :1", use(id);