add and use 'skip_if_windows' helper method

- add a new helper method to skip tests if on Windows platform
  - skip those tests that fail due to lack of support for symlinked files
    on Windows.

test/helper.rb

@@ -158,4 +158,11 @@ class JekyllUnitTest < Minitest::Test
       str
     )
   end
+
+  def skip_if_windows(msg = nil)
+    if Utils::Platforms.really_windows?
+      msg ||= "Jekyll does not currently support this feature on Windows."
+      skip msg.to_s.magenta
+    end
+  end
 end

test/test_collections.rb

@@ -186,6 +186,9 @@ class TestCollections < JekyllUnitTest
     end
 
     should "include the symlinked file from site.source in the list of docs" do
+      # no support for including symlinked file on Windows
+      skip_if_windows "Jekyll does not currently support symlinks on Windows."
+
       assert_includes @collection.docs.map(&:relative_path), "_methods/um_hi.md"
     end
   end

test/test_entry_filter.rb

@@ -82,6 +82,9 @@ class TestEntryFilter < JekyllUnitTest
 
     # rubocop:disable Performance/FixedSize
     should "include only safe symlinks in safe mode" do
+      # no support for symlinks on Windows
+      skip_if_windows "Jekyll does not currently support symlinks on Windows."
+
       site = Site.new(site_configuration("safe" => true))
       site.reader.read_directories("symlink-test")
 
@@ -91,6 +94,9 @@ class TestEntryFilter < JekyllUnitTest
     # rubocop:enable Performance/FixedSize
 
     should "include symlinks in unsafe mode" do
+      # no support for symlinks on Windows
+      skip_if_windows "Jekyll does not currently support symlinks on Windows."
+
       site = Site.new(site_configuration)
 
       site.reader.read_directories("symlink-test")

test/test_theme.rb

@@ -55,6 +55,9 @@ class TestTheme < JekyllUnitTest
     end
 
     should "return the resolved path when a symlink & resolved path exists" do
+      # no support for symlinks on Windows
+      skip_if_windows "Jekyll does not currently support symlinks on Windows."
+
       expected = File.expand_path("./_layouts", @expected_root)
       assert_equal expected, @theme.send(:path_for, :_symlink)
     end