fix test for go 1.16 (magefile#330)

* fix test for 1.16

Author: natefinch

.travis.yml

@@ -9,6 +9,7 @@ branches:
 # library, I'm not going to worry about older versions for now.
 go:
  - tip
+ - 1.16beta1
  - 1.15.x
  - 1.14.x
  - 1.13.x

mage/main_test.go

@@ -1436,6 +1436,18 @@ func Test() {
 }
 stderr.Reset()
 stdout.Reset()
+
+// we need to run go mod tidy, since go build will no longer auto-add dependencies.
+cmd = exec.Command("go", "mod", "tidy")
+cmd.Dir = dir
+cmd.Env = os.Environ()
+cmd.Stderr = stderr
+cmd.Stdout = stdout
+if err := cmd.Run(); err != nil {
+t.Fatalf("Error running go mod tidy: %v\nStdout: %s\nStderr: %s", err, stdout, stderr)
+}
+stderr.Reset()
+stdout.Reset()
 code := Invoke(Invocation{
 Dir: dir,
 Stderr: stderr,