df: port the new df test to POSIX sed, larger file systems

df: port the new df test to POSIX sed, larger file systems

* tests/df/df-output.sh: For the test "df -B1K --output=size",
do not assume that the file system size fits in 9 bytes; it
might be larger than that, so omit leading space.  Also, use
portable 'sed' commands: POSIX says sed commands inside { } should
all end in newline.