When are all those tests taking place?
A lot of these tests are done as component tests (i.e. unit tests in the S/W world). As soon as the tooling is done, they start running the tests on those items, but remember that many of the components are off-the-shelf (OTS) as in existing components from other camera models. The testing of the final, completed camera body would begin as soon as the first fully-assembled body rolls off the line. Some would go out for field testing (World Cup?), others would go into the automated testing with robots pushing the shutter 100,000+ times, weather simulation rooms, impact tests, etc. The automated testing takes much less time that you'd think with the robotics and specialized testing labs.
EOS 1D X, 5DIII, M + EF 24 f/1.4II, 50 f/1.2, 85 f/1.2II, 300 f/2.8 IS II || 16-35 f/4 IS, 24-70 f/2.8II, 70-200 f/2.8II || TS-E 17 f/4, 24 f/3.5II || M 22 f/2, 18-55 f/3.5-5.6 IS || 1.4x III, 2x III