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

toolchain/parse/testdata/basics/multifile.carbon

@@ -4,6 +4,12 @@
 //
 // AUTOUPDATE
 
+// --- a.carbon
+fn A() {}
+
+// --- b.carbon
+fn B() {}
+
 // CHECK:STDOUT: - filename: a.carbon
 // CHECK:STDOUT:   parse_tree: [
 // CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
@@ -24,8 +30,3 @@
 // CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 6},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]
-// --- a.carbon
-fn A() {}
-
-// --- b.carbon
-fn B() {}