test: consider default GOEXPERIMENT when matching build tags
test: consider default GOEXPERIMENT when matching build tags

If GOEXPERIMENT environment variable is unset, use the default
value that is baked into the toolchain (instead of no
experiments).

Change-Id: I41f863e6f7439f2d53e3ebd25a7d9cf4a176e32e
Reviewed-on: https://go-review.googlesource.com/c/go/+/309333
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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