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 {