testsuite: Mark T15633a and T15633b as fragile on Windows

As noted in #16813, these tests seem to be fragile on Windows.

testsuite/tests/ghci/should_run/all.T

@@ -46,7 +46,7 @@ test('T15369',     just_ghci, ghci_script, ['T15369.script'])
 
 test('T15633a',
      [extra_files(['tc-plugin-ghci/']),
-      when(opsys('mingw32'), multi_cpu_race),
+      when(opsys('mingw32'), [multi_cpu_race, fragile(16813)]),
       only_ways(['ghci']),
       pre_cmd('$MAKE -s --no-print-directory -C tc-plugin-ghci package.plugins01 TOP={top}'),
       extra_hc_opts("-package-db tc-plugin-ghci/pkg.plugins01/local.package.conf -fplugin TcPluginGHCi")
@@ -55,7 +55,7 @@ test('T15633a',
 
 test('T15633b',
      [extra_files(['tc-plugin-ghci/']),
-      when(opsys('mingw32'), multi_cpu_race),
+      when(opsys('mingw32'), [multi_cpu_race, fragile(16813)]),
       only_ways(['ghci']),
       pre_cmd('$MAKE -s --no-print-directory -C tc-plugin-ghci package.plugins01 TOP={top}'),
       extra_hc_opts("-package-db tc-plugin-ghci/pkg.plugins01/local.package.conf")