Exclude test cases

Test cases in `TestObjSpace` and `TestRubyOptimization` are related to
the specific behaviors of the default GC.  Specifically, they test the
amount of the off-heap malloc allocation, which is different when using
MMTk.

Test cases in `TestGc` are related to the GC count which is not yet
implemented.

Author: wks

test/.excludes-mmtk/TestGc.rb

@@ -1,3 +1,5 @@
+exclude(:test_count, "GC count not yet implemented")
+exclude(:test_gc_disabled_start, "GC count not yet implemented")
 exclude(:test_expand_heap, "testing behaviour specific to default GC")
 exclude(:test_gc_config_disable_major, "testing behaviour specific to default GC")
 exclude(:test_gc_config_disable_major_gc_start_always_works, "testing behaviour specific to default GC")
@@ -22,4 +24,5 @@
 exclude(:test_stat_heap, "testing behaviour specific to default GC")
 exclude(:test_stat_heap_all, "testing behaviour specific to default GC")
 exclude(:test_stat_heap_constraints, "testing behaviour specific to default GC")
+exclude(:test_stat_single, "GC count not yet implemented.  Actually GC can be triggered between evaluation.")
 exclude(:test_thrashing_for_young_objects, "testing behaviour specific to default GC")

test/.excludes-mmtk/TestObjSpace.rb

@@ -1,3 +1,6 @@
 exclude(:test_dump_all_full, "testing behaviour specific to default GC")
 exclude(:test_dump_flags, "testing behaviour specific to default GC")
 exclude(:test_finalizer, "times out in debug mode on Ubuntu")
+exclude(:test_memsize_of, "testing behaviour specific to default GC")
+exclude(:test_memsize_of_root_shared_string, "testing behaviour specific to default GC")
+exclude(:test_dump_includes_slot_size, "testing behaviour specific to default GC")

test/.excludes-mmtk/TestRubyOptimization.rb

@@ -0,0 +1 @@
+exclude(:test_string_freeze_saves_memory, "testing behaviour specific to default GC")