Fix frozen string literal warning in InstanceMethod

I've mostly fixed this by using `Object.new` instead of instances of
`String` to act as partial mocks.

I've also had to disable one test which is specifically stubbing a
method on an instance of `String` when the Ruby version is v3.4 or
higher. This is OK, because we already have behaviour in place to
prevent stubbing of a method on a frozen object [1,2], so the behaviour
under test would be moot at that point.

Previously I was seeing a lot of the following warnings when running
the tests under Ruby v3.4:

    lib/mocha/instance_method.rb:16: warning: literal string will be frozen in the future

[1]: https://github.com/freerange/mocha/blob/5c7d14cb7d779bd22da3dbfe9d16ed1091ace7a1/lib/mocha/object_methods.rb#L75-L77
[2]: https://github.com/freerange/mocha/blob/5c7d14cb7d779bd22da3dbfe9d16ed1091ace7a1/lib/mocha/object_methods.rb#L121-L123

test/acceptance/failure_messages_test.rb

@@ -52,11 +52,13 @@ def test_should_display_mock_address_when_expectation_was_on_unnamed_mock
     assert_match Regexp.new("#<Mock:#{OBJECT_ADDRESS_PATTERN}>"), test_result.failures[0].message
   end
 
-  def test_should_display_string_when_expectation_was_on_string
-    test_result = run_as_test do
-      'Foo'.expects(:bar)
+  unless Mocha::RUBY_V34_PLUS
+    def test_should_display_string_when_expectation_was_on_string
+      test_result = run_as_test do
+        'Foo'.expects(:bar)
+      end
+      assert_match Regexp.new(%("Foo")), test_result.failures[0].message
     end
-    assert_match Regexp.new(%("Foo")), test_result.failures[0].message
   end
 
   def test_should_display_that_block_was_expected

test/acceptance/partial_mocks_test.rb

@@ -13,8 +13,8 @@ def teardown
 
   def test_should_pass_if_all_expectations_are_satisfied
     test_result = run_as_test do
-      partial_mock_one = 'partial_mock_one'
-      partial_mock_two = 'partial_mock_two'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
 
       partial_mock_one.expects(:first)
       partial_mock_one.expects(:second)
@@ -29,8 +29,8 @@ def test_should_pass_if_all_expectations_are_satisfied
 
   def test_should_fail_if_all_expectations_are_not_satisfied
     test_result = run_as_test do
-      partial_mock_one = 'partial_mock_one'
-      partial_mock_two = 'partial_mock_two'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
 
       partial_mock_one.expects(:first)
       partial_mock_one.expects(:second)

test/acceptance/sequence_block_test.rb

@@ -76,8 +76,8 @@ def test_should_allow_invocations_in_sequence_even_if_expected_on_different_mock
 
   def test_should_constrain_invocations_to_occur_in_expected_order_even_if_expected_on_partial_mocks
     test_result = run_as_test do
-      partial_mock_one = '1'
-      partial_mock_two = '2'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
 
       sequence('one') do
         partial_mock_one.expects(:first)
@@ -93,8 +93,8 @@ def test_should_constrain_invocations_to_occur_in_expected_order_even_if_expecte
 
   def test_should_allow_invocations_in_sequence_even_if_expected_on_partial_mocks
     test_result = run_as_test do
-      partial_mock_one = '1'
-      partial_mock_two = '2'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
 
       sequence('one') do
         partial_mock_one.expects(:first)

test/acceptance/sequence_test.rb

@@ -72,8 +72,8 @@ def test_should_allow_invocations_in_sequence_even_if_expected_on_different_mock
 
   def test_should_constrain_invocations_to_occur_in_expected_order_even_if_expected_on_partial_mocks
     test_result = run_as_test do
-      partial_mock_one = '1'
-      partial_mock_two = '2'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
       sequence = sequence('one')
 
       partial_mock_one.expects(:first).in_sequence(sequence)
@@ -88,8 +88,8 @@ def test_should_constrain_invocations_to_occur_in_expected_order_even_if_expecte
 
   def test_should_allow_invocations_in_sequence_even_if_expected_on_partial_mocks
     test_result = run_as_test do
-      partial_mock_one = '1'
-      partial_mock_two = '2'
+      partial_mock_one = Object.new
+      partial_mock_two = Object.new
       sequence = sequence('one')
 
       partial_mock_one.expects(:first).in_sequence(sequence)