Support the go1.21 testing.Testing() function (#4190)

**What type of PR is this?**

Feature

**What does this PR do? Why is it needed?**

This change adds in the necessary flags in `go test` to allow
[`testing.Testing`] to work as expected. According to the [upstream
source], it is set with a `-X` option in the `go` tooling, so this
change makes `rules_go` operate accordingly.

This should be safe for older Go versions too, given that the link
command [docs](https://pkg.go.dev/cmd/link) say that the `-X` arg is not
"effective" when the target variable is not defined in the code.

Tests are also provided, and `buildifier` moved around some unrelated
lines. I'd be happy to undo the unrelated changes.

[`testing.Testing`]: https://pkg.go.dev/testing#Testing
[upstream source]:
https://cs.opensource.google/go/go/+/refs/tags/go1.21.0:src/testing/testing.go;l=647-661

**Which issues(s) does this PR fix?**

Fixes #3686.
Closes #4096

**Other notes for review**

This is intended to replace #4096, which seems to be inactive. The
functional code change in this PR was done at the same location.

go/private/rules/test.bzl

@@ -155,6 +155,12 @@ def _go_test_impl(ctx):
     # in bzltestutil/init.go.
     test_gc_linkopts.extend(["-X", "+initfirst/github.com/bazelbuild/rules_go/go/tools/bzltestutil/chdir.RunDir=" + run_dir])
 
+    # This is needed for the testing.Testing() function to work in go
+    # 1.21+.  See
+    # https://cs.opensource.google/go/go/+/refs/tags/go1.21.0:src/testing/testing.go;l=647-661
+    # for more details.
+    test_gc_linkopts.extend(["-X", "testing.testBinary=1"])
+
     # Now compile the test binary itself
     test_deps = external_archive.direct + [external_archive] + ctx.attr._testmain_additional_deps
     if go.coverage_enabled:

tests/core/go_binary/BUILD.bazel

@@ -1,3 +1,5 @@
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+load("@bazel_skylib//rules:run_binary.bzl", "run_binary")
 load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")
 load("@io_bazel_rules_go//go/tools/bazel_testing:def.bzl", "go_bazel_test")
 load(":linkmode.bzl", "linkmode_pie_wrapper")
@@ -241,7 +243,27 @@ go_binary(
 go_binary(
     name = "meaning2",
     srcs = [
-        "//tests/core/go_library:use_syso_srcs",
         "meaning2.go",
+        "//tests/core/go_library:use_syso_srcs",
     ],
 )
+
+# These three verify that testing.Testing() returns `false` in a
+# `go_binary`.
+go_binary(
+    name = "testing_testing_bin",
+    srcs = ["testing_testing.go"],
+)
+
+run_binary(
+    name = "testing_testing_bin_run",
+    outs = ["testing_testing_bin_run.out"],
+    args = ["$(location :testing_testing_bin_run.out)"],
+    tags = ["manual"],
+    tool = ":testing_testing_bin",
+)
+
+build_test(
+    name = "testing_testing_test",
+    targets = [":testing_testing_bin_run"],
+)

tests/core/go_binary/testing_testing.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+	"os"
+)
+
+func main() {
+	if testing.Testing() {
+		panic("testing.Testing() returned 'true' in a binary")
+	}
+
+	file, err := os.Create(os.Args[1])
+	if err != nil {
+		panic(fmt.Sprintf("Failed to open output file %s", err))
+	}
+	file.Close()
+}

tests/core/go_test/BUILD.bazel

@@ -208,6 +208,12 @@ test_suite(
     tests = ["same_package_{}_test".format(i) for i in range(1, 80)],
 )
 
+# Verifies that testing.Testing() is true in a `go_test`.
+go_test(
+    name = "testing_testing_test",
+    srcs = ["testing_testing_test.go"],
+)
+
 go_bazel_test(
     name = "testmain_without_exit_test",
     srcs = ["testmain_without_exit_test.go"],
@@ -289,7 +295,7 @@ go_test(
 go_test(
     name = "syso_direct_test",
     srcs = [
-        "//tests/core/go_library:use_syso_srcs",
         "syso_direct_test.go",
+        "//tests/core/go_library:use_syso_srcs",
     ],
 )

tests/core/go_test/testing_testing_test.go

@@ -0,0 +1,11 @@
+package testing_testing
+
+import (
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	if ! testing.Testing() {
+		panic("testing.Testing() returned 'false' in a test")
+	}
+}