fix redundant MySQL display sizes and add tests

[elcreator]

Hello!

• Type: bug fix
• Link to issue: https://github.com/phalcon/migrations/issues/139

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the Contributing Guidelines
• [x] I have checked that another pull request for this purpose does not exist
• [x] I wrote some tests for this PR

Small description of change: In MySQL8+ the numeric fields size (which is a display size 1-255 characters used in some clients and not the size of bytes used for a field) is redundant and so deprecated.