Fix addKey to handle the case where a key is being added, and that key
is the leading substring of an already existing key within that file.
For example: add "server=192.168.1.1", when "server=192.168.1.178"
already exists within the /etc/dnsmasq.d/01-pihole.conf file.
Check pihole docker with PIHOLE_DNS="192.168.1.178;192.168.1.1". Its
/etc/dnsmasq/01-pihole.conf will be missing its second server= entry.
Add the test_key_addition_substr, to test addKey when
its adding a substring key of an existing key in the file.
Signed-off-by: William Blew <william@kulian.org>
b9a6970bfd