os: use relative paths in a test dir in TestOpenError
Refactor TestOpenError to use relative paths in test cases,
in preparation for extending it to test os.Root.
Use a test temporary directory instead of system directory
with presumed-known contents.
Move the testcase type and case definitions inline with the test.
For #67002
Change-Id: Idc53dd9fcecf763d3e4eb3b4643032e3003d7ef4
Reviewed-on: https://go-review.googlesource.com/c/go/+/620157 Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>