Mark plugins-external as broken on Windows

This test is broken on Windows, so we explicitly mark it as such now
that we stop skipping plugin tests on Windows.

(cherry picked from commit a72015d6)

testsuite/tests/plugins/all.T

@@ -9,11 +9,6 @@ setTestOpts([
     # sequentially until this is fixed.  This likely means that #13194 isn't fully
     # solved.  See also #15313.
     when(opsys('mingw32'), multi_cpu_race),
-    # Unfortunately even with the above these tests are incredibly flaky on Windows.
-    # In any given testsuite run at least half a dozen fail for one reason of another
-    # (typically a cmmbination of timeouts, some missing static archive errors from ld,
-    # and a few plain crashes).
-    when(opsys('mingw32'), fragile(16405))
 ])
 
 
@@ -103,13 +98,11 @@ test('plugins15',
 
 test('T10420',
      [extra_files(['rule-defining-plugin/']),
-      when(opsys('mingw32'), expect_broken(21322)),
       pre_cmd('$MAKE -s --no-print-directory -C rule-defining-plugin package.T10420 TOP={top}')],
      makefile_test, [])
 
 test('T10294',
      [extra_files(['annotation-plugin/']),
-
       pre_cmd('$MAKE -s --no-print-directory -C annotation-plugin package.T10294 TOP={top}')],
      makefile_test, [])
 
@@ -262,13 +255,11 @@ test('T20218b',
 
 test('test-defaulting-plugin',
      [extra_files(['defaulting-plugin/']),
-      when(opsys('mingw32'), fragile(21293)),
       pre_cmd('$MAKE -s --no-print-directory -C defaulting-plugin package.test-defaulting-plugin TOP={top}')],
      makefile_test, [])
 
 test('test-defaulting-plugin-fail',
      [extra_files(['defaulting-plugin/']),
-      when(opsys('mingw32'), fragile(21293)),
       pre_cmd('$MAKE -s --no-print-directory -C defaulting-plugin package.test-defaulting-plugin-fail TOP={top}')],
      makefile_test, [])
 
@@ -319,7 +310,7 @@ test('test-echo-in-line-many-args',
 test('plugins-external',
      [extra_files(['shared-plugin/']),
       pre_cmd('$MAKE -s --no-print-directory -C shared-plugin package.plugins01 TOP={top}'),
-      when(opsys('linux') and not ghc_dynamic(), expect_broken(20706))],
+      when(opsys('mingw32') or (opsys('linux') and not ghc_dynamic()), expect_broken(20706))],
      makefile_test, [])
 
 test('test-phase-hooks-plugin',