Add --reverse flag to wait until the port is no longer available

[gordalina]

This program is great at waiting for a service to become available, especially when we are coordinating programs that depend on network resources. Conversely, said programs sometimes need to wait for network resources to be closed or unavailable in order to shutdown gracefully.

This is where the --reverse flag comes in, it reverses the test that is being performed on the network resource. Allowing for the use case above to work.

This Pull Request also adds the accompanying test case.

[deiga]

I think that this adds unnecessary complexity to the app.
Could you achieve the same functionality by using a while or until loop in your script?

[gordalina]

The purpose of adding this to this program is to reduce complexity in the script that consumes it, which I believe to be the main reason to use wait-for-it: not to have to write the same error-prone code over and over for a given script/service.

[gordalina]

@douglas-gibbons can you review this PR? Thank you.

[atkrad]

Hey @gordalina

You can use the Wait4X, It's already supported the reverse checking.

Example:
wait4x tcp 127.0.0.1:80 --invert-check