Currently, the top-level testsuite always uses whatever version
of Go is found in the PATH to execute all the tests. This
forces the developers to tweak the PATH to run the testsuite.

Change it to use the same version of Go used to run run.go.
This allows developers to run the testsuite using the tip
compiler by simply saying "../bin/go run run.go".

I think this is a better solution compared to always forcing
"../bin/go", because it allows developers to run the testsuite
using different Go versions, for instance to check if a new
test is fixed in tip compared to the installed compiler.

Fixes #24217

Change-Id: I41b299c753b6e77c41e28be9091b2b630efea9d2
Reviewed-on: https://go-review.googlesource.com/98439
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>