Reenable truffleruby in CI and omit the 2 failing tests

* See #73

Author: eregon

.github/workflows/test.yml

@@ -40,7 +40,6 @@ jobs:
       run: bundle install
     - name: Run test
       run: rake compile test
-      continue-on-error: ${{ contains(matrix.ruby, 'truffleruby') }}
 
   spec:
     runs-on: ubuntu-latest

test/pathname/test_pathname.rb

@@ -491,6 +491,7 @@ def obj.to_str; "a/b"; end
   end
 
   def test_initialize_nul
+    omit "https://github.com/truffleruby/truffleruby/issues/4047" if RUBY_ENGINE == "truffleruby"
     assert_raise(ArgumentError) { Pathname.new("a\0") }
   end
 
@@ -612,6 +613,7 @@ def test_destructive_update
   end
 
   def test_null_character
+    omit "https://github.com/truffleruby/truffleruby/issues/4047" if RUBY_ENGINE == "truffleruby"
     assert_raise(ArgumentError) { Pathname.new("\0") }
   end