os: relax tests and add examples for UserCacheDir and UserConfigDir

Per https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html:
“If, when attempting to write a file, the destination directory is
non-existent an attempt should be made to create it with permission
0700. […] The application should be prepared to handle the case where
the file could not be written […]. In such case it may choose to
present an error message to the user.”

In certain CI environments, these directories have well-defined
locations but do not exist and cannot be created. In that case,
we now choose to log and return from the test without failing it.

To prevent the functions from falling back to being entirely untested,
we still fail the test (and “present an error message to the user”) if
either function returns an empty string without an error, or returns a
path that refers to a non-directory or results in an error other than
ErrNotExist.

In addition, since the tests themselves no longer create subdirectories,
we add examples illustrating the suggested pattern of usage.

Fixes #64990.

Change-Id: Ie72106424f5ebe36eaf9288c22710d74bb14a462
Reviewed-on: https://go-review.googlesource.com/c/go/+/554815
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

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