Fix flaky tests on Windows

[FedericoPonzi]

ref: #518 (comment)
I'll run CI a few times and fix any flakiness that comes in the way.

• Add retries for pdflatex installs on windows - sometimes the upstream goes into timeout. Apparently there is no native retry support.
• checkModelCancel was not stubbing parseSpec, causing timeout errors at times when sany was to slow to spawn and finish run.
• the initial sany load in divergence test is quite slow reaching the timeout. Next load takes 3s, and 300ms thereafter. Increased the timeout. This is just an hypothesis, I'm geniunly surprised it takes >15s to finish run that test.