[GR-38054] Disable GC test failing on native, due to the count increasing by 2 on native instead of 1 per Full GC

Author: eregon

test/mri/excludes/TestGc.rb

@@ -12,3 +12,4 @@
 exclude :test_start_full_mark, "needs investigation"
 exclude :test_start_immediate_sweep, "needs investigation"
 exclude :test_verify_internal_consistency, "needs investigation"
+exclude :test_gc_disabled_start, "fails on native: GR-38054"