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/artifacts/source.map.amd.txt

@@ -1 +0,0 @@
-{"version":3,"sources":["test"],"names":["compiler","main","container","depth0","helpers","partials","data","useData"],"mappings":"kHAAA,CAAAA,SAAA,CAAA,EAAA,YAAAC,KAAA,SAAAC,EAAAC,EAAAC,EAAAC,EAAAC,GAAA,MAAA,gBAAAC,SAAA"}