gnu: tar: Disable troublesome tests on the Hurd.

gnu: tar: Disable troublesome tests on the Hurd.

The sparse03.at test apparently freezes the Hurd; messages
on the console:

165: listing sparse files bigger than 2^33

ext2fs: device:hd0s1: warning: bit already cleared for block 245780
ext2fs: device:hd0s1: warning: bit already cleared for block 245781
ext2fs: device:hd0s1: warning: bit already cleared for block 245782
...
ext2fs: BUG: unexpected fault on disk image ... err 0xa
ext2fs: disk-pager.c:98: fault_handler: Assertion 'err' failed.

the other tests just fail.

* gnu/packages/base.scm (tar)[arguments]: When compiling for the Hurd,
skip `sparse' and failing tests.