test: add basic tests to the TM_ARG_ env support. (#1910)

## What this PR does / why we need it:

Some tests for the #1850 PR.

Note: tests are failing because it's not rebased on top of #1850 as
original PR contained conflicts at the time I did this.

## Which issue(s) this PR fixes:
none

## Special notes for your reviewer:

## Does this PR introduce a user-facing change?
```
no, just tests
```

e2etests/core/cli_args_env_test.go

@@ -0,0 +1,129 @@
+// Copyright 2024 Terramate GmbH
+// SPDX-License-Identifier: MPL-2.0
+
+package core_test
+
+import (
+	"path/filepath"
+	"regexp"
+	"testing"
+
+	. "github.com/terramate-io/terramate/e2etests/internal/runner"
+	"github.com/terramate-io/terramate/test/sandbox"
+)
+
+func TestBasicArgEnvFlags(t *testing.T) {
+	t.Parallel()
+
+	s := sandbox.NoGit(t, true)
+	s.BuildTree([]string{
+		`s:stacks/a`,
+		`s:stacks/b`,
+		`s:stacks/c`,
+		`s:stacks/a/1`,
+		`s:stacks/a/2`,
+		`s:stacks/a/3`,
+
+		// not a stack
+		`d:stacks/d`,
+	})
+
+	t.Run("TM_ARG_CHDIR", func(t *testing.T) {
+		t.Parallel()
+
+		type testcase struct {
+			env      []string
+			expected string
+		}
+
+		for _, tc := range []testcase{
+			{
+				expected: nljoin(
+					"stacks/a",
+					"stacks/a/1",
+					"stacks/a/2",
+					"stacks/a/3",
+					"stacks/b",
+					"stacks/c",
+				),
+			},
+			{
+				env: []string{"TM_ARG_CHDIR=."},
+				expected: nljoin(
+					"stacks/a",
+					"stacks/a/1",
+					"stacks/a/2",
+					"stacks/a/3",
+					"stacks/b",
+					"stacks/c",
+				),
+			},
+			{
+				env: []string{"TM_ARG_CHDIR=stacks"},
+				expected: nljoin(
+					"a",
+					"a/1",
+					"a/2",
+					"a/3",
+					"b",
+					"c",
+				),
+			},
+			{
+				env: []string{"TM_ARG_CHDIR=stacks/a"},
+				expected: nljoin(
+					".",
+					"1",
+					"2",
+					"3",
+				),
+			},
+			{
+				env: []string{"TM_ARG_CHDIR=" + filepath.Join(s.RootDir(), "stacks/a")},
+				expected: nljoin(
+					".",
+					"1",
+					"2",
+					"3",
+				),
+			},
+			{
+				env: []string{"TM_ARG_CHDIR=stacks/d"},
+			},
+		} {
+			tc := tc
+			tmcli := NewCLI(t, "", tc.env...)
+			tmcli.SetWorkingDir(s.RootDir())
+			AssertRunResult(t,
+				tmcli.Run("list"),
+				RunExpected{
+					Stdout: tc.expected,
+				},
+			)
+		}
+	})
+
+	t.Run("TM_ARG_RUN_NO_RECURSIVE=true", func(t *testing.T) {
+		t.Parallel()
+
+		// boolean flags accepts "true" and "false"
+		tmcli := NewCLI(t, s.RootDir(), "TM_ARG_RUN_NO_RECURSIVE=true")
+		AssertRunResult(t,
+			tmcli.Run("run", "--", HelperPath, "echo", "hello"),
+			RunExpected{
+				Status:      1,
+				StderrRegex: regexp.QuoteMeta(`Error: --no-recursive provided but no stack found`),
+			},
+		)
+
+		// boolean flags also accepts "0" and "1"
+		tmcli = NewCLI(t, s.RootDir(), "TM_ARG_RUN_NO_RECURSIVE=1")
+		AssertRunResult(t,
+			tmcli.Run("run", "--", HelperPath, "echo", "hello"),
+			RunExpected{
+				Status:      1,
+				StderrRegex: regexp.QuoteMeta(`Error: --no-recursive provided but no stack found`),
+			},
+		)
+	})
+}

e2etests/internal/runner/runner.go

@@ -36,6 +36,7 @@ type (
 		environ   []string
 		AppendEnv []string
 
+		wd      string
 		userDir string
 	}
 
@@ -124,6 +125,12 @@ func NewInteropCLI(t *testing.T, chdir string, env ...string) CLI {
 	return tm
 }
 
+// SetWorkingDir sets the process working directory. Not to be confused with tm.Chdir
+// which controls the `--chdir` flag.
+func (tm *CLI) SetWorkingDir(dir string) {
+	tm.wd = dir
+}
+
 // PrependToPath prepend the provided directory to the OS's PATH
 // environment variable in a portable way.
 func (tm *CLI) PrependToPath(dir string) {
@@ -232,6 +239,7 @@ func (tm CLI) NewCmd(args ...string) *Cmd {
 	cmd.Stderr = stderr
 	cmd.Stdin = stdin
 	cmd.Env = env
+	cmd.Dir = tm.wd
 
 	return &Cmd{
 		t:      t,