net/http: enable WASM fetch where supported

The existing check was introduced to allow tests to pass
on WASM without an environment where the fetch RoundTripper
could run. However, the check now prohibits the use of the
Fetch RoundTripper in all WASM tests, even where the
RoundTripper could run. The new change should only disable
the RoundTripper when used in an environment without fetch.

Fixes #32289

Change-Id: I30d2e0dbcb0e64d4b1a46b583f7e984c2a57d5e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/179118


Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>