Add spec for source class

spec/source_spec.cr

@@ -0,0 +1,28 @@
+require "./spec_helper"
+
+describe Spectator::Source do
+  describe "#file" do
+    it "is the expected value" do
+      file = __FILE__
+      source = Spectator::Source.new(file, __LINE__)
+      source.file.should eq(file)
+    end
+  end
+
+  describe "#line" do
+    it "is the expected value" do
+      line = __LINE__
+      source = Spectator::Source.new(__FILE__, line)
+      source.line.should eq(line)
+    end
+  end
+
+  describe "#to_s" do
+    it "is formatted correctly" do
+      file = __FILE__
+      line = __LINE__
+      source = Spectator::Source.new(file, line)
+      source.to_s.should eq("#{file}:#{line}")
+    end
+  end
+end