Externalize the test table

I'm beginning work on the file reading functions, so I need some files
to read in my tests. I'll also need the WholeFile struct to compare
against.

The input string has been moved out into a file and put back into the
test fixture with `include_str!()`. The WholeFile construction has been
moved to a util function so I can reuse it in another test.

test_data/sample_config.toml

@@ -0,0 +1,15 @@
+[all]
+gitea_url = "http://localhost:3000"
+token = "fake-token"
+
+["/home/robert/projects/gt-tool"]
+owner = "robert"
+repo = "gt-tool"
+
+["/home/robert/projects/rcalc"]
+owner = "jamis"
+repo = "rcalc"
+
+["/home/robert/projects/rcalc-builders"]
+owner = "jamis"
+repo = "rcalc"