archive/tar: skip TestSparseFiles on Plan 9

CL 60871 added TestSparseFiles. This test is succeeding
on Plan 9 when executed on the ramfs file system, but
is failing when executed on the Fossil file system.

This may be due to an issue in the handling of sparse
files in the Fossil file system on Plan 9 that should
be investigated.

Updates #21977.

Change-Id: I177afff519b862a5c548e094203c219504852006
Reviewed-on: https://go-review.googlesource.com/65352


Reviewed-by: Ian Lance Taylor <iant@golang.org>