[dev.typeparams] test: add string quoting support to test/run.go

This CL copies go/build's splitQuoted function (used for parsing #cgo
directives within `import "C"` preambles) to parse test recipe
commands. In particular, this now allows writing "build" and "run"
tests that use -gcflags to pass multiple compiler flags.

Change-Id: I0d18a9c13a4ce24bbdfa1da8662c0498c93a6762
Reviewed-on: https://go-review.googlesource.com/c/go/+/327275


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>