Testing suite for verifying new commits

[muzimuzhi]

Brief outline of the proposed feature

A comprehensive testing suite was once discussed in #666, but this issue wants to discuss a non-comprehensive one, consisted of accumulated per-issue/feature tests.

The problems addressed by new commits are probably not covered by simply compiling pgfmanual, and even if covered, might not throw errors if a regression appears. So a PDF-based tests is much better. I've tried to write several l3build tests and think pgf can use it. Missing features ranging from DVI-based tests (maybe with the help of dviasm) to running a test only with specific engines do exist, but current utilities can already cover 80%+ use cases.

Considering the complex structure of pgf-tikz itself, some directory-based frameworks as well as helper macros are needed.