test: make it easier to run tests for subsystems

[bcoe]

Problem statement

• it would be nice to be able to more easily run tests across a given
  subsystem, e.g., http, when their tests are spread out across
  multiple folders (pummel/, sequential/, etc.).
• it would be nice to be able to report on coverage for a subset of
  tests/subsystems, speeding up the test coverage feedback loop.

Potential solution: split tests into subsystem-specific folders

this was proposed in #15437.

It became apparent in #15437, and in the discussion surrounding it, that
splitting tests into subsystem-specific folders potentially creates more
problems than it presents benefits:

• you end up changing the git-history of 1000s of files.
• at a glance, there's even more of a paradox of choice with regards
  to what folder a test should be placed in (what if a test interacts
  with two subsystems?).

Proposed solution: introduce suites_<subsystem> shorthand, make CI_JS_SUITES CI_NATIVE_SUITES configurable

Rather than reorganizing tests into an alternative folder structure, this
pull request introduces the concept of a suites_<subsystem> shorthand to
test.py.

Running:

$ python tools/test.py -J --mode=release suites_child-process

Would result in any tests matching the */test-child-process-* pattern being
executed, regardless of whether they are contained in test/sequential,
test/parallel, etc.

Other changes

• CI_JS_SUITES and CI_NATIVE_SUITES are now configurable, this makes it
  possible to more easily run a coverage report against a subset of tests.
• the default suites run by tools/test.py now uses a black-list rather than
  a white-list; this makes it harder to miss adding a new test folder to CI.
• the known_issues suite of tests is no longer run as part of coverage; my
  concern being that we should count a line as covered, if it's exercised
  by a failing test.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test,build

CC: @addaleax, @gibfahn @jasnell