Add tests for more class A/B/C-based cases

[phil-davis]

1) Add check_ipv4_display_output function so that common text and regex stuff that validates output format is in a single place 2) Make existing display_ipv4 tests use check_ipv4_display_output 3) check_ipv4_display_output used "local -n", and the "-n" turned out to require Bash 4.3. The default Travis image is still an "old" Ubuntu that has an older Bash version. It turned out that Travis have a newer Ubuntu image called "trusty" available, which does have Bash 4.3.. So we might as well move up to that, rather than messing about on the old image to fetch a newer bash. 4) Add new test cases for an ordinary subnet in each class A/B/C with subnet size that matches the (old-fashioned) class subnet mask, the first subnet in the class (some of these are special reserved subnets), and a subnet of the 1st and 2nd subnets in the class (which are the cases where the resulting subnet overlaps a reserved and unreserved section). That tests quite a few variations of the descriptive text that is on the last line of output.

[phil-davis]

Blog about newer Travis image: https://blog.travis-ci.com/2016-11-08-trusty-container-public-beta for information.

[troglobit]

Thank you so much for your hard work! :tada:

Good point about Travis there. I actually read that blog the other day, for another project. Thank you for upping the version to Trusty.