embed/internal/embedtest: use parenthesized vars for one test
embed/internal/embedtest: use parenthesized vars for one test

The gofrontend code mishandled this case, so add it to the test.

Change-Id: I183b8fab57552320b04c4826c590b7c6d36b6548
Reviewed-on: https://go-review.googlesource.com/c/go/+/358836
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,nawawi/go