encoding/json: improve fidelity of TestUnmarshal for Numbers
In particular, cover the behavior of unmarshaling a JSON string
into a Number type regardless of whether the `string` option
is specified or not.
Change-Id: Ibc55f16860442240bcfeea1fd51aaa76f7e50f67
Reviewed-on: https://go-review.googlesource.com/c/go/+/641416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>