Enabled frozen string literals (#1154)

* Enabled frozen string literals

* Update rubocop config

* Prefer string interpolation in simple cases

Co-Authored-By: Dylan Thacker-Smith <dylan.smith@shopify.com>

test/unit/block_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class BlockUnitTest < Minitest::Test
@@ -61,7 +63,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -79,7 +81,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf

test/unit/condition_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class ConditionUnitTest < Minitest::Test

test/unit/context_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class HundredCentes

test/unit/file_system_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class FileSystemUnitTest < Minitest::Test

test/unit/i18n_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class I18nUnitTest < Minitest::Test

test/unit/lexer_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class LexerUnitTest < Minitest::Test

test/unit/parser_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class ParserUnitTest < Minitest::Test

test/unit/partial_cache_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class PartialCacheUnitTest < Minitest::Test

test/unit/regexp_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class RegexpUnitTest < Minitest::Test

test/unit/strainer_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class StrainerUnitTest < Minitest::Test

test/unit/tag_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class TagUnitTest < Minitest::Test
@@ -31,7 +33,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -49,7 +51,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf

test/unit/tags/case_tag_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class CaseTagUnitTest < Minitest::Test

test/unit/tags/for_tag_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class ForTagUnitTest < Minitest::Test

test/unit/tags/if_tag_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class IfTagUnitTest < Minitest::Test

test/unit/template_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class TemplateUnitTest < Minitest::Test

test/unit/tokenizer_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class TokenizerTest < Minitest::Test

test/unit/variable_unit_test.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'test_helper'
 
 class VariableUnitTest < Minitest::Test