[flakes] ensure all golang tests pass when flakes flag is enabled (#697)

## Summary

Needed to make two fixes:

1. `RmTest`:
here, the test was failing because with flakes flag we now check that
the user
has the package installed previously and so we do `devbox run <command>`
to trigger
package installation as a side-effect.

2. `ShellTest`:
Flakes no longer works with there being no default shell, so we call
that out. But
in a subsequent PR, I'll revisit this since I think we can get rid of
this block.
Wanted to keep the scope of this PR to just enabling these tests to
pass.

## How was it tested?

enabled the Flakes feature flag in the code and ran `go test ./...`

examples/testdata/go/go-1.19/devbox.json

@@ -11,4 +11,4 @@
   "nixpkgs": {
     "commit": "52e3e80afff4b16ccb7c52e9f0f5220552f03d04"
   }
-}
+}

internal/boxcli/rm_test.go

@@ -16,20 +16,29 @@ func TestRm(t *testing.T) {
 			"hello"
 		],
 		"shell": {
+		  "scripts": {
+			"test1": "echo test1"
+		  },
 		  "init_hook": null
 		},
 		"nixpkgs": {
 		  "commit": "af9e00071d0971eb292fd5abef334e66eda3cb69"
 		}
 	}`
-	td := testframework.Open()
-	defer td.Close()
-	err := td.SetDevboxJSON(devboxJSON)
+	testbox := testframework.Open()
+	defer testbox.Close()
+	err := testbox.SetDevboxJSON(devboxJSON)
 	assert.NoError(t, err)
-	output, err := td.RunCommand(RemoveCmd(), "hello")
+
+	// First, run a devbox script to install the packages as a side-effect
+	_, err = testbox.RunCommand(RunCmd(), "test1")
+	assert.NoError(t, err)
+
+	// Now, run the Remove command
+	output, err := testbox.RunCommand(RemoveCmd(), "hello")
 	assert.NoError(t, err)
 	assert.Contains(t, output, "hello (hello-2.12.1) is now removed.")
-	devboxjson, err := td.GetDevboxJSON()
+	devboxjson, err := testbox.GetDevboxJSON()
 	assert.NoError(t, err)
 	assert.NotContains(t, devboxjson.RawPackages, "hello")
 }

internal/boxcli/shell_test.go

@@ -16,6 +16,8 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"go.jetpack.io/devbox/internal/boxcli/featureflag"
+	"go.jetpack.io/devbox/internal/nix"
 	"go.jetpack.io/devbox/internal/testframework"
 )
 
@@ -311,6 +313,13 @@ func TestShell(t *testing.T) {
 	err := td.SetDevboxJSON(devboxJSON)
 	assert.NoError(t, err)
 	output, err := td.RunCommand(ShellCmd())
-	assert.NoError(t, err)
-	assert.Contains(t, output, "Starting a devbox shell...")
+	if featureflag.Flakes.Enabled() {
+		assert.Error(t, err)
+		if !errors.Is(err, nix.ErrNoDefaultShellUnsupportedInFlakesMode) {
+			assert.Fail(t, "Expected error %s but received %s", nix.ErrNoDefaultShellUnsupportedInFlakesMode, err)
+		}
+	} else {
+		assert.NoError(t, err)
+		assert.Contains(t, output, "Starting a devbox shell...")
+	}
 }

internal/nix/shell.go

@@ -41,6 +41,8 @@ const (
 	shPosix   name = "posix"
 )
 
+var ErrNoDefaultShellUnsupportedInFlakesMode = errors.New("No default shell not supported in Flakes mode")
+
 // Shell configures a user's shell to run in Devbox. Its zero value is a
 // fallback shell that launches a regular Nix shell.
 type Shell struct {
@@ -205,8 +207,10 @@ func (s *Shell) Run(nixShellFilePath, nixFlakesFilePath string) error {
 	// Launch a fallback shell if we couldn't find the path to the user's
 	// default shell.
 	if s.binPath == "" {
+
+		// TODO savil: fix this.
 		if featureflag.Flakes.Enabled() {
-			return errors.New("No default shell not supported in Flakes mode")
+			return ErrNoDefaultShellUnsupportedInFlakesMode
 		}
 		cmd := exec.Command("nix-shell", "--pure")
 		cmd.Args = append(cmd.Args, toKeepArgs(env, buildAllowList(s.env))...)