add test for #7636

testsuite/tests/rts/T7636.hs

+import GHC.Conc.Sync
+import System.Environment
+
+test n = atomically $ f [1..n]
+ where
+  f [] = retry
+  f (x:xs) = do
+    ys <- f xs
+    return (x:ys)
+
+main = do
+  [n] <- getArgs
+  test (read n)

testsuite/tests/rts/T7636.stderr

+T7636: thread blocked indefinitely in an STM transaction

testsuite/tests/rts/all.T

@@ -162,3 +162,5 @@ test('T7040_ghci', [ only_ways(['ghci']),
 test('T7227', [ extra_run_opts('+RTS -tT7227.stat --machine-readable -RTS'),
                 extra_clean(['T7227.stat']) ]
             , compile_and_run, [''] )
+
+test('T7636', [ exit_code(1), extra_run_opts('100000') ], compile_and_run, [''] )