The Go file go1.20.14:test/run.go added more handling of the test flag arguments. Since some of the new tests for go1.20 use these flags, I updated the flag parsing into the GopherJS copy of the same runner, gorepo/run.go. These updates are nearly identical copies of the code from the Go repo. Any deviations I've commented with a // GOPHERJS: comment. This is part of #1270

Other than being able to handle more flags, the only main change was adding the timeout flag ability. The comment in the timeout (i.e. "This command-timeout code adapted...") is from the original source code.

Here is an example of a test, fixedbugs/issue59334.go, failing prior to this update:

# go run run.go -- fixedbugs/issue59334.go
exit status 1
unknown shorthand flag: 't' in -tags=purego

FAIL    fixedbugs/issue59334.go 0.037s