@betterC, @TestBetterC , @betterCTest or @("betterC") to annotate Phobos's unittests?

[Seb]

Phobos has recently gotten a primitive way to run betterC tests 
(https://github.com/dlang/phobos/pull/6640).

Now, it would be really cool if we can annotate existing tests 
with e.g. `@betterCTest` and thus ensure that those tests work 
with -betterC (i.e. extract them and run them with a minimal test 
runner.

a) @betterC

Probably too popular and would lead to too many conflicts.

b) @TestBetterC
c) @betterCTest || @bettercTest

Would probably be defined a enum somewhere in `std.internal`

d) @("betterC")

A bit ugly and harder to work with. If we ever want to name the 
unittests in Phobos, this will make it harder.

At some point we might also want to test whether something runs 
even without any runtime 
(https://github.com/dlang/phobos/pull/6641), so e.g. 
`@baremetalTest` might be defined too.

My favorite is (c) - any objections?

See also: https://github.com/dlang/tools/pull/357 (implementation 
of the test extraction filter for UDAs)