Fix random test failures in TestExcerpt #to_liquid. (#8884)

Merge pull request 8884

test/test_filters.rb

@@ -37,6 +37,7 @@ class TestFilters < JekyllUnitTest
   context "filters" do
     setup do
       @sample_time = Time.utc(2013, 3, 27, 11, 22, 33)
+      @timezone_before_test = ENV["TZ"]
       @filter = make_filter_mock(
         "timezone"               => "UTC",
         "url"                    => "http://example.com",
@@ -55,6 +56,10 @@ class TestFilters < JekyllUnitTest
       ]
     end
 
+    teardown do
+      ENV["TZ"] = @timezone_before_test
+    end
+
     should "markdownify with simple string" do
       assert_equal(
         "<p>something <strong>really</strong> simple</p>\n",