Skip struct test in case that Struct isn't implemented

test/t/struct.rb

 ##
 # Struct ISO Test
 
-assert('Struct', '15.2.18') do
+if StructEnabled
+  assert('Struct', '15.2.18') do
     Struct.class == Class
-end
+  end
 
-assert('Struct superclass', '15.2.18.2') do
+  assert('Struct superclass', '15.2.18.2') do
     Struct.superclass == Object
-end
+  end
 
-assert('Struct.new', '15.2.18.3.1') do
+  assert('Struct.new', '15.2.18.3.1') do
     c = Struct.new(:m1, :m2)
     c.superclass == Struct and
       c.members == [:m1,:m2]
-end
+  end
 
-assert('Struct#[]', '15.2.18.4.2') do
+  assert('Struct#[]', '15.2.18.4.2') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     cc[:m1] == 1 and cc["m2"] == 2
-end
+  end
 
-assert('Struct#[]=', '15.2.18.4.3') do
+  assert('Struct#[]=', '15.2.18.4.3') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     cc[:m1] = 3
     cc[:m1] == 3
-end
+  end
 
-assert('Struct#each', '15.2.18.4.4') do
+  assert('Struct#each', '15.2.18.4.4') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     a = []
@@ -36,9 +37,9 @@ assert('Struct#each', '15.2.18.4.4') do
       a << x
     }
     a[0] == 1 and a[1] == 2
-end
+  end
 
-assert('Struct#each_pair', '15.2.18.4.5') do
+  assert('Struct#each_pair', '15.2.18.4.5') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     a = []
@@ -46,16 +47,18 @@ assert('Struct#each_pair', '15.2.18.4.5') do
       a << [k,v]
     }
     a[0] == [:m1, 1] and a[1] == [:m2, 2]
-end
+  end
 
-assert('Struct#members', '15.2.18.4.6') do
+  assert('Struct#members', '15.2.18.4.6') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     cc.members == [:m1,:m2]
-end
+  end
 
-assert('Struct#select', '15.2.18.4.7') do
+  assert('Struct#select', '15.2.18.4.7') do
     c = Struct.new(:m1, :m2)
     cc = c.new(1,2)
     cc.select{|v| v % 2 == 0} == [2]
+  end
 end
+