Fix StringConcatenation offenses (non-production)

Prefer interpolation over concatenation

```
Comparison ("String#{'String'}" vs 'String' + 'String'):
       interpolation: 11821321.0 i/s
       concatenation:  8849491.7 i/s - 1.34x  (± 0.00) slower
```

2 files changed, 2 insertions, 2 deletions

diff --git a/test/workbook/tc_shared_strings_table.rb b/test/workbook/tc_shared_strings_table.rb
index 2edf5f4a..4ac8b112 100644
--- a/test/workbook/tc_shared_strings_table.rb
+++ b/test/workbook/tc_shared_strings_table.rb
@@ -45,7 +45,7 @@ class TestSharedStringsTable < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "sharedStirngs.xml Invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "sharedStirngs.xml Invalid#{errors.map(&:message)}")
   end
 
   def test_remove_control_characters_in_xml_serialization
diff --git a/test/workbook/worksheet/tc_rich_text_run.rb b/test/workbook/worksheet/tc_rich_text_run.rb
index 6274dda8..be05d818 100644
--- a/test/workbook/worksheet/tc_rich_text_run.rb
+++ b/test/workbook/worksheet/tc_rich_text_run.rb
@@ -151,7 +151,7 @@ class RichTextRun < Test::Unit::TestCase
 
   def test_multiline_autowidth
     wrap = @p.workbook.styles.add_style({ alignment: { wrap_text: true } })
-    awtr = Axlsx::RichTextRun.new('I\'m bold' + "\n", b: true)
+    awtr = Axlsx::RichTextRun.new("I'm bold\n", b: true)
     rt = Axlsx::RichText.new
     rt.runs << awtr
     @ws.add_row [rt], style: wrap