tests: add extension to sed -i for GNU/BSD sed compatibility

BSD sed requires that an extension is provided to the -i (in-place edit)
flag, which may immediately follow the -i or may be separated by a space -
sed -i .bak and sed -i.bak are equivalent.  The extension is optional with
GNU sed, but if provided must immediately follow the -i.  Thus, sed -i.bak
behaves identically with both GNU and BSD sed.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20191114203615.2866-1-emaste@freefall.freebsd.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>