Add specs for Frame#context

spec/backtracer/backtrace/frame_spec.cr

@@ -23,4 +23,39 @@ describe Backtracer::Backtrace::Frame do
       end
     end
   end
+
+  {% unless flag?(:release) || !flag?(:debug) %}
+    describe "#context" do
+      it "returns proper lines" do
+        with_configuration do |configuration|
+          with_backtrace(caller) do |backtrace|
+            backtrace.frames.first.tap do |first_frame|
+              context_lines = configuration.context_lines.should_not be_nil
+              context = first_frame.context.should_not be_nil
+
+              lines = File.read_lines(__FILE__)
+              lineidx = context.lineno - 1
+
+              context.pre
+                .should eq(lines[Math.max(0, lineidx - context_lines), context_lines]?)
+              context.line
+                .should eq(lines[lineidx]?)
+              context.post
+                .should eq(lines[Math.min(lines.size, lineidx + 1), context_lines]?)
+            end
+          end
+        end
+      end
+
+      it "returns given amount of lines" do
+        with_backtrace(caller) do |backtrace|
+          backtrace.frames.first.tap do |first_frame|
+            context = first_frame.context(3).should_not be_nil
+            context.pre.size.should eq(3)
+            context.post.size.should eq(3)
+          end
+        end
+      end
+    end
+  {% end %}
 end