Add unit tests for sources.go

Tests cover most of the cache and download related code paths and specify the expected result of various starting states and external failure modes.
Where the current code's behaviour doesn't match a test's expectations, the test is disabled and annotated with a TODO until it can be fixed.
Added dependency on `github.com/powerman/check` and ran `go mod vendor`.

.gitattributes

@@ -1,3 +1,6 @@
 *               text=auto
 *.go            text        diff=golang
 *.bat           text        eol=crlf
+
+# Ensure test fixtures don't get mangled
+**/testdata/**  -text