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.