tests/util/grub-shell-luks-tester: Fix detached header test getting wrong header...

tests/util/grub-shell-luks-tester: Fix detached header test getting wrong header path

When $detached_header was set 1, $luksdiskfile was set to the LUKS header
file path with "${detached_header:-$luksfile}" appended, which evaluates
to "1". Fix this by using two statements to set $luksdiskfile. The first
sets it to the header file if $detached_header is set, otherwise leave it
unset. The second statement sets it to itself if it is already set,
otherwise it is set to $luksfile.

Fixes: a7b540e6e (tests: Add cryptomount functional test)
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>