test-sendfiles: Feed `lzma -cd' stdin rather than a filename.

test-sendfiles: Feed `lzma -cd' stdin rather than a filename.

Håkon Alstadheim reported that Debian's lzma-9.22-2 would complain at
`lzma -cd foo.tar' with `unknown suffix -- unchanged' even though it was
not being asked to alter the file, or produce a new one based on its
filename.  This appears to be https://bugs.debian.org/700681.  The lzma
from Arch Linux's xz 5.2.3-1 does no have this fault.  Work around it by
feeding the file to decompress on standard input for all the compression
programs being tested;  none of the others should mind.