libstdc++: Improve testing for path::operator+=(const string&)

	* testsuite/27_io/filesystem/path/concat/strings.cc: Test more cases.

From-SVN: r279111

libstdc++-v3/ChangeLog

 2019-12-09  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/27_io/filesystem/path/concat/strings.cc: Test more cases.
+
 	PR libstdc++/92853
 	* src/c++17/fs_path.cc (filesystem::path::operator+=(const path&)):
 	Do not process a trailing directory separator twice.

libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc

@@ -113,10 +113,29 @@ test03()
   p4 += s;
   compare_paths(p4, path(s0+'/'+s));
 }
+
+void
+test04()
+{
+  // Concat every test path onto every test path.
+  for (path p : __gnu_test::test_paths)
+  {
+    for (path x : __gnu_test::test_paths)
+    {
+      auto prior_native = p.native();
+      p += x.native();
+      VERIFY( p.native() == prior_native + x.native() );
+      path copy(p);
+      compare_paths( copy, p );
+    }
+  }
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }