Add fold vs. mconcat test T17123

testsuite/tests/perf/should_run/T17123.hs

+import qualified Data.Text as T
+import Data.Foldable
+
+xs :: [T.Text]
+xs = [ T.pack $ show n | n <- [0..10000] ]
+
+main :: IO ()
+main = do
+    fold xs `seq` return ()

testsuite/tests/perf/should_run/all.T

@@ -372,6 +372,11 @@ test('T15185',
     compile_and_run,
     ['-O'])
 
+test('T17123',
+    [collect_stats('bytes allocated', 20), only_ways(['normal'])],
+    compile_and_run,
+    ['-O'])
+
 # Test performance of creating Uniques.
 test('UniqLoop',
      [collect_stats('bytes allocated',5),