add tests about is_interesting_file

spec/util_spec.cr

@@ -35,6 +35,26 @@ describe "compare_numerically" do
   end
 end
 
+describe "is_interesting_file" do
+  it "returns true when filename has interesting file extension" do
+    filename = "manga.cbz"
+    result = is_interesting_file filename
+    result.should eq true
+  end
+
+  it "returns false if file extension is not interesting" do
+    filename = "info.json"
+    result = is_interesting_file filename
+    result.should eq false
+  end
+
+  it "returns true when fileext has uppercase" do
+    filename = "manga.ZiP"
+    result = is_interesting_file filename
+    result.should eq true
+  end
+end
+
 describe "chapter_sort" do
   it "sorts correctly" do
     ary = ["Vol.1 Ch.01", "Vol.1 Ch.02", "Vol.2 Ch. 2.5", "Ch. 3", "Ch.04"]