Run compilation tests against Ruby 3.3.0-preview2

Before compiling the native gems against the latest, unreleased version
of Ruby, check the gem compiles and passes its tests when the user has
to compile against it on install.

Note 3.3.0-preview2 is not available for Windows.

See #108

.github/workflows/tests.yml

@@ -70,6 +70,7 @@ jobs:
     strategy:
       matrix:
         ruby:
+          - '3.3.0-preview2'
           - '3.2'
           - '3.1'
           - '3.0'
@@ -124,7 +125,7 @@ jobs:
       fail-fast: false
       matrix:
         runs-on: ["ubuntu-latest", "macos-latest"]
-        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2"]
+        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3.0-preview2"]
         include:
           - ruby: "2.6"
             runs-on: "windows-2019"