cmd/go: fix a portability issue in the cd script command

Currently all script tests use the UNIX path separator with the cd
command, causing the PWD environment variable to have the incorrect path
separator on Windows.

Call filepath.FromSlash on the cd command argument.
Update the testdata/script/README to document that the cd argument must
use slashes.

Add a regression test.

To reproduce this issue, a test must use the cd command followed by a
stdout or stderr command containing the pattern $PWD.

Change-Id: Ib2dc5f185cc2476451402787996d14df91f7dddb
Reviewed-on: https://go-review.googlesource.com/c/go/+/319311
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/script_test.go

@@ -517,7 +517,7 @@ func (ts *testScript) cmdCd(want simpleStatus, args []string) {
 		ts.fatalf("usage: cd dir")
 	}
 
-	dir := args[0]
+	dir := filepath.FromSlash(args[0])
 	if !filepath.IsAbs(dir) {
 		dir = filepath.Join(ts.cd, dir)
 	}

src/cmd/go/testdata/script/README

@@ -102,6 +102,7 @@ The commands are:
 
 - cd dir
   Change to the given directory for future commands.
+  The directory must use slashes as path separator.
 
 - chmod perm path...
   Change the permissions of the files or directories named by the path arguments

src/cmd/go/testdata/script/test_script_cmdcd.txt

@@ -0,0 +1,13 @@
+# Tests that after a cd command, where usually the UNIX path separator is used,
+# a match against $PWD does not fail on Windows.
+
+cd $WORK/a/b/c/pkg
+
+go list -find -f {{.Root}}
+stdout $PWD
+
+-- $WORK/a/b/c/pkg/go.mod --
+module pkg
+
+-- $WORK/a/b/c/pkg/pkg.go --
+package pkg