Repository: go
Issue: net/http/httptest: Server.Close mutates http.DefaultTransport
Labels: NeedsInvestigation

### Go version

go version go1.21.6 linux/amd64

### Output of `go env` in your module/workspace:

```shell
$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/akashem/.cache/go-build'
GOENV='/home/akashem/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/akashem/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/akashem/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.6'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/akashem/go/src/k8s.io/kubernetes/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2922644753=/tmp/go-build -gno-record-gcc-switches'
```


### What did you do?

It seems that the default transport object `http.DefaultTransport` gets mutated by the `Close` method of `httptest.Server`. 

Here is a test that demonstrates it:
```
func TestHTTPTestServerCloseMutatesDefaultTransport(t *testing.T) {
	server := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("pong"))
	}))
	server.EnableHTTP2 = true
	server.StartTLS()

	client := server.Client()
	client.Get(server.URL + "/ping")

	want := http.DefaultTransport.(*http.Transport).TLSClientConfig
	if want != nil {
		t.Errorf("expected TLSClientConfig of http.DefaultTransport to be nil")
	}
	server.Close()
	if got := http.DefaultTransport.(*http.Transport).TLSClientConfig; want != got {
		t.Errorf("TLSClientConfig has been mutated")
	}
}
```

The `TLSClientConfig` field of the `DefaultTransport` object is `nil` to start with, but then it gets replaced with a non nil instance after the Close method returns. Here is the call chain of the mutation:

`httptest/Server.Close` invokes the `CloseIdleConnections` method of  the default `DefaultTransport` object:
https://github.com/golang/go/blob/cf52e709977d331a70df9463cf9e307024b6779f/src/net/http/httptest/server.go#L237-L242

https://github.com/golang/go/blob/cf52e709977d331a70df9463cf9e307024b6779f/src/net/http/transport.go#L783-L784

`TLSClientConfig.NextProtos` gets set:
https://github.com/golang/go/blob/cf52e709977d331a70df9463cf9e307024b6779f/src/net/http/h2_bundle.go#L7269-L7274

I am not certain if it's common for production code to exercise:
```
	if t, ok := http.DefaultTransport.(closeIdleTransport); ok {
		t.CloseIdleConnections()
	}
```

I ran into this issue with unit tests that verify that the `TLSClientConfig` field of a given transport does not get mutated unexpectedly. I can change the test to work around it. But I wanted to bring this to your attention so you can assess whether it can impact production code.