testing: implement t.Setenv

This method has been added in Go 1.17 and is used in archive/zip
starting with Go 1.20. Therefore, this method is now needed in Go 1.20.

I've left out the parts that disable parallel execution of tests,
because we don't do that in TinyGo.

See:
* https://github.com/golang/go/issues/41260
* https://go-review.googlesource.com/c/go/+/260577

src/testing/testing.go

@@ -117,6 +117,7 @@ type TB interface {
 	Log(args ...interface{})
 	Logf(format string, args ...interface{})
 	Name() string
+	Setenv(key, value string)
 	Skip(args ...interface{})
 	SkipNow()
 	Skipf(format string, args ...interface{})
@@ -330,6 +331,27 @@ func (c *common) TempDir() string {
 	return dir
 }
 
+// Setenv calls os.Setenv(key, value) and uses Cleanup to
+// restore the environment variable to its original value
+// after the test.
+func (c *common) Setenv(key, value string) {
+	prevValue, ok := os.LookupEnv(key)
+
+	if err := os.Setenv(key, value); err != nil {
+		c.Fatalf("cannot set environment variable: %v", err)
+	}
+
+	if ok {
+		c.Cleanup(func() {
+			os.Setenv(key, prevValue)
+		})
+	} else {
+		c.Cleanup(func() {
+			os.Unsetenv(key)
+		})
+	}
+}
+
 // runCleanup is called at the end of the test.
 func (c *common) runCleanup() {
 	for {

src/testing/testing_test.go

@@ -138,3 +138,60 @@ func testTempDir(t *testing.T) {
 		t.Errorf("unexpected files in TempDir")
 	}
 }
+
+func TestSetenv(t *testing.T) {
+	tests := []struct {
+		name               string
+		key                string
+		initialValueExists bool
+		initialValue       string
+		newValue           string
+	}{
+		{
+			name:               "initial value exists",
+			key:                "GO_TEST_KEY_1",
+			initialValueExists: true,
+			initialValue:       "111",
+			newValue:           "222",
+		},
+		{
+			name:               "initial value exists but empty",
+			key:                "GO_TEST_KEY_2",
+			initialValueExists: true,
+			initialValue:       "",
+			newValue:           "222",
+		},
+		{
+			name:               "initial value is not exists",
+			key:                "GO_TEST_KEY_3",
+			initialValueExists: false,
+			initialValue:       "",
+			newValue:           "222",
+		},
+	}
+
+	for _, test := range tests {
+		if test.initialValueExists {
+			if err := os.Setenv(test.key, test.initialValue); err != nil {
+				t.Fatalf("unable to set env: got %v", err)
+			}
+		} else {
+			os.Unsetenv(test.key)
+		}
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Setenv(test.key, test.newValue)
+			if os.Getenv(test.key) != test.newValue {
+				t.Fatalf("unexpected value after t.Setenv: got %s, want %s", os.Getenv(test.key), test.newValue)
+			}
+		})
+
+		got, exists := os.LookupEnv(test.key)
+		if got != test.initialValue {
+			t.Fatalf("unexpected value after t.Setenv cleanup: got %s, want %s", got, test.initialValue)
+		}
+		if exists != test.initialValueExists {
+			t.Fatalf("unexpected value after t.Setenv cleanup: got %t, want %t", exists, test.initialValueExists)
+		}
+	}
+}