Very rough code that can helm install

The recommended way to test code that uses exec.Cmd involves setting up
a real exec.Cmd that invokes `go test` with additional arguments that
fire off a specially-constructed test that behaves the way the mocked-
out script would be expected to do. It's a sensible way to test exec.Cmd
itself, but for code that merely invokes it, I think it makes more sense
to use actual mocks.

internal/run/cmd.go

@@ -0,0 +1,55 @@
+package run
+
+import (
+	"io"
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+// The cmd interface provides a generic form of exec.Cmd so that it can be mocked out in tests.
+type cmd interface {
+	// methods that exist on exec.Cmd
+	Output() ([]byte, error)
+	Run() error
+	CombinedOutput() ([]byte, error)
+	Start() error
+	StderrPipe() (io.ReadCloser, error)
+	StdinPipe() (io.WriteCloser, error)
+	StdoutPipe() (io.ReadCloser, error)
+	String() string
+	Wait() error
+
+	// setters for struct fields set by callers of exec.Cmd
+	Path(string)
+	Args([]string)
+	Env([]string)
+	Dir(string)
+	Stdin(io.Reader)
+	Stdout(io.Writer)
+	Stderr(io.Writer)
+	ExtraFiles([]*os.File)
+	SysProcAttr(*syscall.SysProcAttr)
+
+	// getters for struct fields generated by exec.Cmd
+	Process() *os.Process
+	ProcessState() *os.ProcessState
+}
+
+// execCmd wraps exec.Cmd along with setters for exec.Cmd's struct fields, implementing the cmd interface.
+type execCmd struct {
+	*exec.Cmd
+}
+
+func (c *execCmd) Path(p string)                      { c.Cmd.Path = p }
+func (c *execCmd) Args(a []string)                    { c.Cmd.Args = a }
+func (c *execCmd) Env(e []string)                     { c.Cmd.Env = e }
+func (c *execCmd) Dir(d string)                       { c.Cmd.Dir = d }
+func (c *execCmd) Stdin(s io.Reader)                  { c.Cmd.Stdin = s }
+func (c *execCmd) Stdout(s io.Writer)                 { c.Cmd.Stdout = s }
+func (c *execCmd) Stderr(s io.Writer)                 { c.Cmd.Stderr = s }
+func (c *execCmd) ExtraFiles(f []*os.File)            { c.Cmd.ExtraFiles = f }
+func (c *execCmd) SysProcAttr(s *syscall.SysProcAttr) { c.Cmd.SysProcAttr = s }
+
+func (c *execCmd) Process() *os.Process           { return c.Cmd.Process }
+func (c *execCmd) ProcessState() *os.ProcessState { return c.Cmd.ProcessState }