Fix remaining JRuby + Windows rubygems test failures

assert_self_install_permissions' ensure block calls File.chmod with
a splatted Dir.glob result; when the omit short-circuits the body,
the glob is empty and JRuby (unlike CRuby) rejects a chmod with no
paths. Guard the ensure to skip when no files were created.

test_generate_bin_symlink_win32 and test_generate_bin_with_dangling_symlink
fall into the "Unable to use symlinks" wrapper branch only when
symlink_supported? is false, but the installer on JRuby/Windows still
manages to create the symlink, so the expected warning never appears.
Omit them on the combo.

Author: hsbt

test/rubygems/test_gem.rb

@@ -201,7 +201,8 @@ def assert_self_install_permissions(format_executable: false, data_mode: 0o640)
     end
     assert_equal(expected, result)
   ensure
-    File.chmod(0o755, *Dir.glob(@gemhome + "/gems/**/"))
+    files = Dir.glob(@gemhome + "/gems/**/")
+    File.chmod(0o755, *files) unless files.empty?
   end
 
   def test_require_missing

test/rubygems/test_gem_installer.rb

@@ -689,8 +689,11 @@ def test_generate_bin_symlink_update_remove_wrapper
 
   def test_generate_bin_symlink_win32
     old_win_platform = Gem.win_platform?
-    Gem.win_platform = true
     old_alt_separator = File::ALT_SEPARATOR
+
+    omit "JRuby on Windows still creates the symlink so the wrapper branch is not exercised" if Gem.win_platform? && Gem.java_platform?
+
+    Gem.win_platform = true
     File.__send__(:remove_const, :ALT_SEPARATOR)
     File.const_set(:ALT_SEPARATOR, "\\")
 
@@ -743,6 +746,8 @@ def test_generate_bin_uses_default_shebang
   end
 
   def test_generate_bin_with_dangling_symlink
+    omit "JRuby on Windows still creates the symlink so the wrapper branch is not exercised" if Gem.win_platform? && Gem.java_platform?
+
     gem_with_dangling_symlink = File.expand_path("packages/ascii_binder-0.1.10.1.gem", __dir__)
 
     installer = Gem::Installer.at(