os: TestChdirAndGetwd doesn't restore initial working directory on GOOS=js GOARCH=wasm

[odeke-em]

Coming here from a test failure that we noticed from @randall77's CL https://go-review.googlesource.com/c/go/+/219143, we got mysterious failures https://go-review.googlesource.com/c/go/+/219143/4#message-18f33eac583b2d9ebefb682fd3d154824e8e5b58 in which even with getting the current working directory totally failed after I had suggested:

Could we perhaps use the current working directory:

   wd, err := Getwd()
   if err != nil {
	t.Fatal(err)
   }
and then
   filepath.Join(wd, ...)

@neelance came in, in https://go-review.googlesource.com/c/go/+/219143/4#message-031f21fab344d60320d779d0ac6b523de9061b22 letting us know that TestChdirAndGetwd is broken on GOOS=js, GOARCH=wasm with

I found the reason: The test TestChdirAndGetwd is unable to restore the initial working directory. This is because fd.Chdir() does not work correctly if the path of fd is relative. This is because NodeJS does not support fchdir so it has to be emulated with chdir.

Here's a possible solution:

	if path[0] != '/' {
		cwd := jsProcess.Call("cwd").String()
		path = cwd + "/" + path
	}
	f := &jsFile{
		path:    path,
		entries: entries,
	}

which is unrelated to Keith's current CL.

I am filing this issue so that we can address that separately and not block Keith's CL.

[cagedmantis]

/cc @rsc @ianlancetaylor @bradfitz @griesemer

[gopherbot]

Change https://golang.org/cl/221717 mentions this issue: syscall: fix Fchdir on js/wasm