test: make default GC behavior "normal"

I'm getting frustrated with long CI run timesxs

CONTRIBUTING.md

@@ -170,7 +170,7 @@ bundle exec rake compile test:memcheck
 ```
 
 
-Note that by default the test suite will run a major GC after each test completes. This has shown to be useful for localizing some classes of memory bugs, but does slow the suite down. Some variations of the test suite behavior are available (see `test/helper.rb` for more info):
+Note that by you can run the test suite with a variety of GC behaviors. For example, running a major after each test completes has, on occasion, been useful for localizing some classes of memory bugs, but does slow the suite down. Some variations of the test suite behavior are available (see `test/helper.rb` for more info):
 
 ``` sh
 # see failure messages immediately

test/helper.rb

@@ -4,10 +4,10 @@
 # Some environment variables that are used to configure the test suite:
 # - NOKOGIRI_TEST_FAIL_FAST: if set to anything, emit test failure messages immediately upon failure
 # - NOKOGIRI_TEST_GC_LEVEL: (roughly in order of stress)
-#   - "normal" - normal GC functionality
+#   - "normal" - normal GC functionality (default)
 #   - "minor" - force a minor GC cycle after each test
-#   - "major" (default for Rubies without compaction) - force a major GC cycle after each test
-#   - "compact" (default for Rubies with compaction) - force a major GC after each test and GC compaction after every 20 tests
+#   - "major" - force a major GC cycle after each test
+#   - "compact" - force a major GC after each test and GC compaction after every 20 tests
 #   - "verify" - force a major GC after each test and verify references-after-compaction after every 20 tests
 #   - "stress" - run tests with GC.stress set to true
 # - NOKOGIRI_GC: read more in test/test_memory_leak.rb
@@ -144,12 +144,8 @@ def initialize_nokogiri_test_gc_level
         "compact"
       elsif (ENV["NOKOGIRI_TEST_GC_LEVEL"] == "verify") && defined?(GC.verify_compaction_references)
         "verify"
-      elsif RUBY_ENGINE == "truffleruby"
-        "normal"
-      elsif defined?(GC.compact)
-        "compact"
       else
-        "major"
+        "normal"
       end
 
       if ["compact", "verify"].include?(@@gc_level)