main: match go test output

This commit makes the output of `tinygo test` similar to that of `go
test`. It changes the following things in the process:

  * Running multiple tests in a single command is now possible. They
    aren't paralellized yet.
  * Packages with no test files won't crash TinyGo, instead it logs it
    in the same way the Go toolchain does.

loader/errors.go

@@ -23,3 +23,13 @@ type Error struct {
 func (e Error) Error() string {
 	return e.Err.Error()
 }
+
+// Error returned when loading a *Program for a test binary but no test files
+// are present.
+type NoTestFilesError struct {
+	ImportPath string
+}
+
+func (e NoTestFilesError) Error() string {
+	return "no test files"
+}