Use mutable strings for mutation tests.

* test/fiddle/test_func.rb (test_string): this test break String buffer
  by `strcpy` ("000" -> "123"). However, the string literal "000" with
  `frozen_string_literal: true` returns a string object from frozen
  string pool. So that after this test "000" from fstring pool becomes
  "123" (modified string). 'test/date/' uses "000" (as fstring) and tests
  are fails (we could check with `make test-all TESTS='fiddle date'`).

* test/fiddle/test_function.rb: ditto.

* test/fiddle/test_import.rb: ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59861 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/fiddle/test_func.rb

@@ -38,7 +38,7 @@ module Fiddle
     def test_string
       stress, GC.stress = GC.stress, true
       f = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
-      buff = "000"
+      buff = +"000"
       str = f.call(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)

test/fiddle/test_function.rb

@@ -60,13 +60,13 @@ module Fiddle
       func = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
 
       assert_nil Fiddle.last_error
-      func.call("000", "123")
+      func.call(+"000", "123")
       refute_nil Fiddle.last_error
     end
 
     def test_strcpy
       f = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
-      buff = "000"
+      buff = +"000"
       str = f.call(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)

test/fiddle/test_import.rb

@@ -128,7 +128,7 @@ module Fiddle
     end
 
     def test_strcpy()
-      buff = "000"
+      buff = +"000"
       str = LIBC.strcpy(buff, "123")
       assert_equal("123", buff)
       assert_equal("123", str.to_s)