update make help to describe everything CI runs & update to newest develop

Description

The top-level goal is to be able to look at continuous integration failures and figure out how to run tests locally. Here are a few points, but the whole thing needs review.

1. How do I reproduce mpirun? Specifically, I do not know how to do that for tests that are failing here:

mpirun -np 1 test/unit/math/mix/fun/abs_test --gtest_output="xml:test/unit/math/mix/fun/abs_test.xml" failed

2. The current documentation refers to the old division of scal vs. mat vs. arr, which no longer exists.

3. The * used in the first line is confusing because it typically means a wildcard, but here it's reused. Write that out in full with an example that refers to the python runtests script, e.g.,

./runTests -j9 test/unit/math/mix/fun/sin_test.cpp

Or say how to run one from make if that's possible.

Current Version:

v3.1.0