add test cases for GC API

test/t/gc.rb

+# Not ISO specified
+
+assert('GC.enable') do
+  GC.disable == false
+  GC.enable == true
+  GC.enable == false
+end
+
+assert('GC.disable') do
+  begin
+    GC.disable == false
+    GC.disable == true
+  ensure
+    GC.enable
+  end
+end
+
+assert('GC.interval_ratio=') do
+  origin = GC.interval_ratio
+  begin
+    (GC.interval_ratio = 150) == 150
+  ensure
+    GC.interval_ratio = origin
+  end
+end
+
+assert('GC.step_ratio=') do
+  origin = GC.step_ratio
+  begin
+    (GC.step_ratio = 150) == 150
+  ensure
+    GC.step_ratio = origin
+  end
+end
+
+assert('GC.generational_mode=') do
+  origin = GC.generational_mode
+  begin
+    (GC.generational_mode = false) == false
+    (GC.generational_mode = true) == true
+    (GC.generational_mode = true) == true
+  ensure
+    GC.generational_mode = origin
+  end
+end