* When using system tools to unpack a .gz file, the input file needs a
different name than the output file. Normally, we generate this new
name by stripping off the .gz extension off of the file name.
This was not sufficient if the file name did not have an extension,
so we temporarily rename the file in that case.
* When using system tar utility to untar on Windows, we were (erroneously)
skipping the actual untar step if the filename was lacking a .tar
extension
* For foo.txz, we were not changing the extension of the decompressed file
(i.e. we would decompress foo.txz to foo.txz). This did not cause any
problems, but is confusing, so has been updated such that the output
filename reflects its decompressed state (i.e. foo.tar).
* Added test for strip_compression_extension
* Update test_native_unpacking to test each archive type with and without
an extension as part of the file name (i.e. we test "foo.tar.gz", but
also make sure we decompress properly if it is named "foo").
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