testsuite: Mark T10672 as broken

This test, which is only run on Windows, seems to be reliably timing
out.

See #16390.

testsuite/tests/rts/T10672/all.T

 test('T10672_x64',
      [extra_files(['Main.hs', 'Printf.hs', 'cxxy.cpp']),
-      [unless(opsys('mingw32'), skip), unless(arch('x86_64'), skip)]],
+      unless(opsys('mingw32'), skip), unless(arch('x86_64'), skip),
+      when(opsys('mingw32'), expect_broken(16390))],
      makefile_test, ['T10672_x64'])
 
 test('T10672_x86',
      [extra_files(['Main.hs', 'Printf.hs', 'cxxy.cpp']),
-      [unless(opsys('mingw32'), skip), unless(arch('i386'), skip)]],
+      unless(opsys('mingw32'), skip), unless(arch('i386'), skip),
+      when(opsys('mingw32'), expect_broken(16390))],
      makefile_test, ['T10672_x86'])