Unify instruction namings also dirs

Was getting confused myself, where it was instruction or instructions, when if the base class was inside or out of dir.
Now dirs are plural, and base class is inside.

test/risc/instructions/test_instructions.rb

@@ -0,0 +1,68 @@
+require_relative "../helper"
+
+module Risc
+  class TestInstructions < MiniTest::Test
+    def setup
+      @label = Label.new("test" , "test",FakeAddress.new(5))
+      @branch = Branch.new("test" , @label)
+      @instruction = Instruction.new("test")
+    end
+    def test_branch_tos1
+      assert @branch.to_s.include?("Branch")
+      assert @branch.to_s.include?("test")
+    end
+    def test_branch_tos2
+      branch = Branch.new(nil ,@label)
+      assert branch.to_s.include?("Branch")
+    end
+    def test_label_tos1
+      assert @label.to_s.include?("Label")
+    end
+    def test_label_tos2
+      assert Label.new("nil",nil,FakeAddress.new(2)).to_s.include?("Label")
+    end
+    def test_last_empty
+      assert_equal @instruction, @instruction.last
+    end
+    def test_last_not_empty
+      @instruction.set_next @branch
+      assert_equal @branch, @instruction.last
+    end
+    def test_append_empty
+      @instruction.append @branch
+      assert_equal @branch, @instruction.last
+    end
+    def test_insert
+      @instruction.insert @branch
+      assert_equal @branch, @instruction.last
+    end
+    def test_insert_two
+      @branch << @label
+      @instruction.insert @branch
+      assert_equal @label , @instruction.last
+    end
+    def test_append_not_empty
+      @instruction.append @branch
+      @instruction.append @label
+      assert_equal @label, @instruction.last
+    end
+    def test_next1
+      assert_nil  @instruction.next
+    end
+    def test_next2
+      @instruction.set_next @label
+      assert_equal @label , @instruction.next
+      assert_nil  @instruction.next(2)
+    end
+    def test_label_is_method
+      label = Label.new("test" , "Object.test" , FakeAddress.new(5))
+      assert label.is_method
+    end
+    def test_label_is_not_method
+      assert ! @label.is_method
+    end
+    def test_insert_self
+      assert_raises {@label.insert(@label)}
+    end
+  end
+end