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/doc_props/tc_app.rb b/test/doc_props/tc_app.rb
index 82e33f12..c066373c 100644
--- a/test/doc_props/tc_app.rb
+++ b/test/doc_props/tc_app.rb
@@ -41,6 +41,6 @@ class TestApp < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "app.xml invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "app.xml invalid#{errors.map(&:message)}")
   end
 end
diff --git a/test/doc_props/tc_core.rb b/test/doc_props/tc_core.rb
index bfacab45..5a04a615 100644
--- a/test/doc_props/tc_core.rb
+++ b/test/doc_props/tc_core.rb
@@ -18,7 +18,7 @@ class TestCore < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "core.xml Invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "core.xml Invalid#{errors.map(&:message)}")
   end
 
   def test_populates_created