test: always run test in unified IR mode regardless of explicit -G flag
test: always run test in unified IR mode regardless of explicit -G flag

Currently, if a test explicitly specify -G flag, if that flag conflict
with compiler default -G flag, the test will be skipped. That's the
reason CL 346469 haven't fixed the unified IR stuff, but still make the
unified builder passed.

This CL makes run.go always run the test in unified IR mode, regardless
of the explicit -G flag specified.

Updates #48094

Change-Id: I959ecaff6aca07844f0ffcf36caa60cf0747e8ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/347271
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

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