unittest: Suite refactor

• Segregate old-style dbtests into their own Suite initialization
  system: OldStyleSuiteSpecification. This is where the funky
  deprecated features can live on without interfering with the
  Suite API. It also gives us a searchable base class to identify
  them in the future.

• OldStyleSuite has `setupTests()` and an `add<T>()` that
  Suite does not have. Suite API can shrink when it doesn't
  have to support these dbtest adaptor features.
  Suite only needs non-template `add(name, callback)`.

• Add OldStyleSuiteInitializer to some dbtests that were missing it!
  These didn't use SuiteInstance to register themselves and were incorrect.
  They would self register, resulting in _allSuites() holding a std::shared_ptr
  to a static-duration Suite object!

• Change `getSuite()` to return `Suite&` instead of `Suite*`.

• No more "self-registering" in Suite constructor. Registration
  must be done as a separate post-construction step. This removes
  some unusual lifetime management code and is easier to document.
  Suite::getSuite(name) is the only way to make a Suite, and it
  does the make_shared and registration calls with a pseudo-private
  ConstructorEnable idiom.

• Suite->run() returns std::unique_ptr<Result> instead of
  raw `Result*`.

• Suite._ran does nothing. Removed.

• Result.cur does nothing. Removed.

• Switch to pass-by-value and std::move for most ctor args.

• Add explicit on 1-arg ctors.

• Get rid of TestHolder. It's just a 2-field struct.

• use fmt instead of snprintf

• TEST and TEST_F macros: generate TEST_TYPE once.
• TEST and TEST_F macros: inline the _agent variable.
• Mark _doRun as `override`.
• Terminology: replace CASE_NAME with SUITE_NAME.

• rename DeathTestImpl -> DeathTestBase
• move getDeathTestPattern into the test as a static member function

• refactor out some repetition from the comparator decl macros

• use constexpr if and diamond relops to clean up the
  ComparisonAssertion class.