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`.

vendor/github.com/powerman/check/util.go

@@ -0,0 +1,64 @@
+package check
+
+import (
+	"fmt"
+	"math"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+)
+
+func callerTestFileLines() (file string, line int, funcLine int) {
+	pc, file, line, ok := runtime.Caller(0)
+	myfile := file
+	for stack := 1; ok && samePackage(myfile, file); stack++ {
+		pc, file, line, ok = runtime.Caller(stack)
+	}
+	if f := runtime.FuncForPC(pc); f != nil {
+		_, funcLine = f.FileLine(f.Entry())
+	}
+	return file, line, funcLine
+}
+
+func samePackage(basefile, file string) bool {
+	return filepath.Dir(basefile) == filepath.Dir(file) && !strings.HasSuffix(file, "_test.go")
+}
+
+func callerFuncName(stack int) string {
+	pc, _, _, _ := runtime.Caller(stack + 1)
+	return strings.TrimPrefix(funcNameAt(pc), "(*C).")
+}
+
+func funcName(f interface{}) string {
+	return funcNameAt(reflect.ValueOf(f).Pointer())
+}
+
+func funcNameAt(pc uintptr) string {
+	name := "<unknown>"
+	if f := runtime.FuncForPC(pc); f != nil {
+		name = f.Name()
+		if i := strings.LastIndex(name, "/"); i != -1 {
+			name = name[i+1:]
+		}
+		if i := strings.Index(name, "."); i != -1 {
+			name = name[i+1:]
+		}
+	}
+	return name
+}
+
+func format(msg ...interface{}) string {
+	if len(msg) > 1 {
+		return fmt.Sprintf(msg[0].(string), msg[1:]...)
+	}
+	return fmt.Sprint(msg...)
+}
+
+// digits return amount of decimal digits in number.
+func digits(number int) int {
+	if number == 0 {
+		return 1
+	}
+	return int(math.Floor(math.Log10(float64(number)) + 1))
+}