Added process stubbing for easier testing of launched subprocesses

[nfx]

Changes

This PR makes unit testing with subprocesses fast.

	ctx := context.Background()
	ctx, stub := process.WithStub(ctx)
	stub.WithDefaultOutput("meeee")

	ctx = env.Set(ctx, "FOO", "bar")

	out, err := process.Background(ctx, []string{"/usr/local/bin/meeecho", "1", "--foo", "bar"})
	require.NoError(t, err)
	require.Equal(t, "meeee", out)
	require.Equal(t, 1, stub.Len())
	require.Equal(t, []string{"meeecho 1 --foo bar"}, stub.Commands())

	allEnv := stub.CombinedEnvironment()
	require.Equal(t, "bar", allEnv["FOO"])
	require.Equal(t, "bar", stub.LookupEnv("FOO"))

This should make further iterations of #914 easier

Tests

make test

[pietern]

I like the idea, but doesn't look finished yet.

[pietern]

LGTM, thanks.

Please update title to something more changelog friendly -- e.g.

Add process stubbing for easier testing