Add testscripts that tests installing 'hello' (#547)

## Summary
Add testscripts that tests installing 'hello'

## How was it tested?
`go test -v`

Author: loreto

internal/nix/run.go

@@ -34,7 +34,12 @@ func RunScript(nixShellFilePath string, projectDir string, cmdWithArgs string, a
 		}
 	}
 
-	cmd := exec.Command("sh", "-c", cmdWithArgs)
+	// Try to find sh in the PATH, if not, default to a well known absolute path.
+	shPath, err := exec.LookPath("sh")
+	if err != nil {
+		shPath = "/bin/sh"
+	}
+	cmd := exec.Command(shPath, "-c", cmdWithArgs)
 	cmd.Env = append(nixEnv, additionalEnv...)
 	cmd.Dir = projectDir
 	cmd.Stdin = os.Stdin

testscripts/basic/install_hello.test.txt

@@ -0,0 +1,21 @@
+# Devbox should be able to install a very simple package like 'hello'
+# and it should work.
+
+# Ensure hello is not found anywhere in the environment
+! exec hello
+! exec devbox run hello
+
+# Initialize devbox
+exec devbox init
+
+# Add the package and run hello with devbox
+exec devbox add hello
+! exec hello
+
+# Run hello and check it prints the right output
+exec devbox run hello
+stdout 'Hello, world!'
+
+# Once we have better progress output, we should check that stderr is empty, with:
+# ! stderr .+  # No stderr output
+# As is, we always print 'Ensuring packages are installed'.

testscripts/testscripts_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/rogpeppe/go-internal/testscript"
 	"github.com/stretchr/testify/require"
 	"go.jetpack.io/devbox/internal/boxcli"
+	"go.jetpack.io/devbox/internal/xdg"
 )
 
 func TestScripts(t *testing.T) {
@@ -75,6 +76,34 @@ func getTestscriptParams(dir string) testscript.Params {
 			oldPath := env.Getenv("PATH")
 			newPath := strings.Split(oldPath, ":")[0]
 			env.Setenv("PATH", newPath)
+
+			// Both devbox itself and nix occasionally create some files in
+			// XDG_CACHE_HOME (which defaults to ~/.cache). For purposes of this
+			// test set it to a location within the test's working directory:
+			cacheHome := filepath.Join(env.WorkDir, ".cache")
+			env.Setenv("XDG_CACHE_HOME", cacheHome)
+			err := os.MkdirAll(cacheHome, 0755) // Ensure dir exists.
+			if err != nil {
+				return err
+			}
+
+			// There is one directory we do want to share across tests: nix's cache.
+			// Without it tests are very slow, and nix would end up re-downloading
+			// nixpkgs every time.
+			// Here we create a shared location for nix's cache, and symlink from
+			// the test's working directory.
+			err = os.MkdirAll(xdg.CacheSubpath("devbox-tests/nix"), 0755) // Ensure dir exists.
+			if err != nil {
+				return err
+			}
+			err = os.Symlink(xdg.CacheSubpath("devbox-tests/nix"), filepath.Join(cacheHome, "nix"))
+			if err != nil {
+				return err
+			}
+
+			// Enable new `devbox run` so we can use it in tests. This is temporary,
+			// and should be removed once we enable this feature flag.
+			env.Setenv("DEVBOX_FEATURE_STRICT_RUN", "1")
 			return nil
 		},
 		Cmds: map[string]func(ts *testscript.TestScript, neg bool, args []string){