Fix Style/RedundantInterpolation offenses

`%` is an operation on `String` that will return a new `String`, so
the interpolation is redundant

Also adds a missing spec on PivotTable#rels_pn

```
IPS:
      uninterpolated:  4045715.7 i/s
        interpolated:  2359775.6 i/s - 1.71x  (± 0.00) slower

Memory:
      uninterpolated:        160 allocated
        interpolated:        232 allocated - 1.45x more
```

1 files changed, 6 insertions, 0 deletions

diff --git a/test/workbook/worksheet/tc_pivot_table.rb b/test/workbook/worksheet/tc_pivot_table.rb
index 606982db..d4db9cef 100644
--- a/test/workbook/worksheet/tc_pivot_table.rb
+++ b/test/workbook/worksheet/tc_pivot_table.rb
@@ -144,6 +144,12 @@ class TestPivotTable < Test::Unit::TestCase
     assert_equal(2, @ws.relationships.size, "adding a pivot table adds a relationship")
   end
 
+  def test_rels_pn
+    @ws.add_pivot_table('G5:G6', 'A1:D5')
+
+    assert_equal("pivotTables/_rels/pivotTable1.xml.rels", @ws.pivot_tables.first.rels_pn)
+  end
+
   def test_to_xml_string
     pivot_table = @ws.add_pivot_table('G5:G6', 'A1:E5', { no_subtotals_on_headers: ['Year'] }) do |pt|
       pt.rows = ['Year', 'Month']