path/filepath: use fsutil with TestEvalSymlinksCanonicalNames

path/filepath: use fsutil with TestEvalSymlinksCanonicalNames

TestEvalSymlinksCanonicalNames fails on system where 8dot3 name creation
is disabled. Add new test that temporarily changes 8dot3 name creation
file system setting and runs TestEvalSymlinksCanonicalNames under that
setting. New test requires administrator access and modifies important
file system setting, so don't run the test unless explicitly requested
by specifying new test flag.

Updates #13980

Change-Id: I598b5b956e6bd0ed556e79d350cb244808c89c0b
Reviewed-on: https://go-review.googlesource.com/20863
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>