tests: stabilize `test-split-legacy-inline-changelog.t` on Windows
The `tar` command is unable to process "C:\path\to\foo.tar" style paths, which
is how `$TESTDIR` is constructed. It also didn't work with
`$TESTDIR_FORWARD_SLASH`- both failed with:
tar: Cannot connect to C: resolve failed
[128]
But `cat` can handle it if the path is quoted, and `tar` can read from stdin.
(binary:application/octet-stream)