.github/workflows/test.yml: reenable truffleruby tests

This reverts the following commits:

 - 25352f4 (Exclude truffleruby with macos-latest, 2023-02-16)
 - d277123 (skip failing test with truffleruby and ubuntu-22.04,
   2023-02-16)

The logs are expired and I was unable to see what exactly was failing,
but these combinations seem to work as expected now.

.github/workflows/test.yml

@@ -23,8 +23,6 @@ jobs:
         exclude:
           - { os: windows-latest, ruby: truffleruby }
           - { os: windows-latest, ruby: truffleruby-head }
-          - { os: macos-latest,   ruby: truffleruby }
-          - { os: ubuntu-20.04,   ruby: truffleruby }
         include:
           - { os: windows-latest, ruby: ucrt }
           - { os: windows-latest, ruby: mswin }