Use tmp directory for files written during tests

- Add spec/tmp directory with .gitkeep file to indicate the folder is intentional
- Add the folder contents to .gitignore
- Use this folder to output the sourcemap file during bin tests. This file is a sideeffect of the main test

spec/expected/source.map.amd.js

@@ -1,2 +1,2 @@
 define(["handlebars.runtime"],function(e){var t=(e=e.default).template;return(e.templates=e.templates||{}).test=t({compiler:[8,">= 4.3.0"],main:function(e,t,a,n,i){return"<div>1</div>"},useData:!0})});
-//# sourceMappingURL=./spec/artifacts/source.map.amd.txt
+//# sourceMappingURL=./spec/tmp/source.map.amd.txt