[ruby/irb] Properly reset USE_COLORIZE after changing it in tests

Some context tests assigns USE_COLORIZE to false and never change it
back. This can potentially affect other tests' result as the default
should be nil (activated) instead.

https://github.com/ruby/irb/commit/986eb16ece

test/irb/test_context.rb

@@ -279,7 +279,6 @@ module TestIRB
     end
 
     def test_omit_on_assignment
-      IRB.conf[:USE_COLORIZE] = false
       input = TestInputMethod.new([
         "a = [1] * 100\n",
         "a\n",
@@ -343,7 +342,7 @@ module TestIRB
     end
 
     def test_omit_multiline_on_assignment
-      IRB.conf[:USE_COLORIZE] = false
+      without_colorize do
         input = TestInputMethod.new([
           "class A; def inspect; ([?* * 1000] * 3).join(%{\\n}); end; end; a = A.new\n",
           "a\n"
@@ -412,12 +411,13 @@ module TestIRB
         assert_equal("", out)
         irb.context.evaluate('A.remove_method(:inspect)', 0)
       end
+    end
 
     def test_echo_on_assignment_conf
       # Default
       IRB.conf[:ECHO] = nil
       IRB.conf[:ECHO_ON_ASSIGNMENT] = nil
-      IRB.conf[:USE_COLORIZE] = false
+      without_colorize do
         input = TestInputMethod.new()
         irb = IRB::Irb.new(IRB::WorkSpace.new(Object.new), input)
 
@@ -439,13 +439,15 @@ module TestIRB
         assert(irb.context.echo?, "echo? should be true by default")
         refute(irb.context.echo_on_assignment?, "echo_on_assignment? should be false when IRB.conf[:ECHO_ON_ASSIGNMENT] is set to false")
       end
+    end
 
     def test_multiline_output_on_default_inspector
       main = Object.new
       def main.inspect
         "abc\ndef"
       end
-      IRB.conf[:USE_COLORIZE] = false
+
+      without_colorize do
         input = TestInputMethod.new([
           "self"
         ])
@@ -468,8 +470,8 @@ module TestIRB
           irb.eval_input
         end
         assert_empty err
-      assert_equal("=> abc\ndef\n",
-                   out)
+        assert_equal("=> abc\ndef\n", out)
+      end
     end
 
     def test_default_return_format
@@ -642,5 +644,15 @@ module TestIRB
           :*, /\b6\n/,
         ], out)
     end
+
+    private
+
+    def without_colorize
+      original_value = IRB.conf[:USE_COLORIZE]
+      IRB.conf[:USE_COLORIZE] = false
+      yield
+    ensure
+      IRB.conf[:USE_COLORIZE] = original_value
+    end
   end
 end