When there are split files, put unattached stdout at the end of the last file. (#3234)

This is consistent with the goal of trying to push unattached stdout to
the bottom of the test file. The current logic had a single file in
mind, and was not adapted for split file logic.

Addresses
https://github.com/carbon-language/carbon-lang/pull/3217#discussion_r1323584624

Note this depends on #3233

testing/file_test/testdata/unattached_multi_file.carbon

@@ -6,11 +6,12 @@
 // CHECK:STDERR: unattached message 3
 // CHECK:STDERR: unattached message 4
 
-// CHECK:STDOUT: 3 args: `default_args`, `a.carbon`, `b.carbon`
-// CHECK:STDOUT: unattached message 1
-// CHECK:STDOUT: unattached message 2
 // --- a.carbon
 aaa
 
 // --- b.carbon
 bbb
+
+// CHECK:STDOUT: 3 args: `default_args`, `a.carbon`, `b.carbon`
+// CHECK:STDOUT: unattached message 1
+// CHECK:STDOUT: unattached message 2