test-libmount: let's see how libmount parses stuff

With libmount-2.33.1-3.fc30.x86_64 I get:
/* test_libmount_unescaping_one escaped space + utf8 */
from '729 38 0:59 / /tmp/\342\200\236zupa\\040z\304\231bowa\342\200\235 rw,relatime shared:395 - tmpfs die\\040Br\303\274he rw,seclabel'
source: 'die Brühe'
source: 'die Br\303\274he'
source: 'die Brühe'
expected: 'die Brühe'
target: '/tmp/„zupa zębowa”'
target: '/tmp/\342\200\236zupa z\304\231bowa\342\200\235'
target: '/tmp/„zupa zębowa”'
expected: '/tmp/„zupa zębowa”'
/* test_libmount_unescaping_one escaped newline */
from '729 38 0:59 / /tmp/x\\012y rw,relatime shared:395 - tmpfs newline rw,seclabel'
source: 'newline'
source: 'newline'
source: 'newline'
expected: 'newline'
target: '/tmp/x
y'
target: '/tmp/x\ny'
target: '/tmp/x
y'
expected: '/tmp/x
y'
/* test_libmount_unescaping_one empty source */
from '760 38 0:60 / /tmp/emptysource rw,relatime shared:410 - tmpfs  rw,seclabel'
source: ''
source: ''
source: ''
expected: ''
target: '/tmp/emptysource'
target: '/tmp/emptysource'
target: '/tmp/emptysource'
expected: '/tmp/emptysource'
/* test_libmount_unescaping_one foo\rbar */
from '790 38 0:61 / /tmp/foo\rbar rw,relatime shared:425 - tmpfs tmpfs rw,seclabel'
source: 'tmpfs'
source: 'tmpfs'
source: 'tmpfs'
expected: 'tmpfs'
target: '/tmp/foo'
target: '/tmp/foo'
target: '/tmp/foo'
expected: 'n/a'

With https://github.com/karelzak/util-linux/issues/780 fixed, we get

/* test_libmount_unescaping_one foo\rbar */
from '790 38 0:61 / /tmp/foo\rbar rw,relatime shared:425 - tmpfs tmpfs rw,seclabel'
source: 'tmpfs'
source: 'tmpfs'
source: 'tmpfs'
expected: 'tmpfs'
target: '/tmp/foo
bar'
target: '/tmp/foo\rbar'
target: '/tmp/foo
bar'
expected: '/tmp/foo
bar'

(cherry picked from commit b57adc94)