tests: port to NFS file servers with clock skew

Several of the tests assumed that a newly created file cannot
have a time stamp dated in the future.  This assumption is not
true when files are served by a remote host whose clock is
slightly in advance of ours.  Fix the problems that I observed
when running "make check" a couple of times on such a server.

* tests/backup01.at: Use --warning=no-timestamp to suppress
clock-skew warnings.
* tests/chtype.at, tests/comprec.at, tests/exclude06.at:
* tests/extrac01.at, tests/extrac03.at, tests/extrac05.at:
* tests/extrac06.at, tests/extrac08.at, tests/extrac13.at:
* tests/extrac14.at, tests/incr01.at, tests/incr03.at, tests/link01.at:
* tests/multiv01.at, tests/multiv02.at, tests/multiv03.at:
* tests/pipe.at, tests/rename02.at, tests/rename03.at:
* tests/rename05.at, tests/same-order01.at, tests/same-order02.at:
* tests/sparse01.at:
Likewise.

tests/exclude06.at

@@ -33,7 +33,7 @@ genfile --length 20 -f test_base_dir[/1.c]
 
 tar cf archive.tar test_base_dir
 mkdir out
-tar -C out -xf archive.tar --exclude='*.txt'
+tar -C out -xf archive.tar --exclude='*.txt' --warning=no-timestamp
 find out -type f
 ],
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