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

[bob-carpenter]

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

[syclik]

Pinging @wds15 and @rok-cesnovar.

[rok-cesnovar]

I can look at this.

[syclik]

Some other things that I'd like to fix with running make help:

• [ ] Reduce the number of lines of output. Right now, it's 113 lines; it's too much for someone to read immediately. We can split this into a few different targets if we need to.
• [ ] The hyperlink at the top is no longer working: > See https://github.com/stan-dev/math/wiki/Developer-Doc#building-and-running-tests for more detail on testing.

I think we can split the help target into multiple targets and have this point which ones to look at.

Since this is just free text that's written to the console, this should be treated as a non-breaking change.