Add the ability to use function calls in @testset directly.

[Seelengrab]

This allows for easier factoring of tests into functions, making it possible to include a test/runtests.jl file without running tests directly (should it be written with this in mind) and enabling running of specific testsets explicitly by calling the functions they're defined in instead.

For example, this PR makes the following work:

f(x) = @test isone(x)
function g(x)
    @test x > 0
    @testset f(x)
end
@testset g(1)

with output:

Test Summary: | Pass  Total
g             |    2      2
Test.DefaultTestSet("g", Any[Test.DefaultTestSet("f", Any[], 1, false, false)], 1, false, false)

If a description string is provided, it will be used instead of the function name.
In case of a failure (e.g. by running @testset g(0)):

g: Test Failed at REPL[11]:2
  Expression: x > 0
   Evaluated: 0 > 0
Stacktrace:
 [1] macro expansion
   @ ~/julia/usr/share/julia/stdlib/v1.8/Test/src/Test.jl:460 [inlined]
 [2] g(x::Int64)
   @ Main ./REPL[11]:2
f: Test Failed at REPL[10]:1
  Expression: isone(x)
Stacktrace:
 [1] macro expansion
   @ ~/julia/usr/share/julia/stdlib/v1.8/Test/src/Test.jl:460 [inlined]
 [2] f(x::Int64)
   @ Main ./REPL[10]:1
Test Summary: | Fail  Total
g             |    2      2
  f           |    1      1
ERROR: Some tests did not pass: 0 passed, 2 failed, 0 errored, 0 broken.

The change was surprisingly minimal and I think it just.. works? Rudimentary testing in the REPL as well as those written tests included here didn't provide any obvious footguns, so I'm inclined to say this is a good idea.

Nevertheless, this is still missing a NEWS.md entry, docs could/should be more extensive and I may have missed something, so I'm opening this as a draft.

[Seelengrab]

Unsure if this is the correct way to deal with getting a string of the function name here.

[Seelengrab]

no idea what's going on with the buildbot/package_* failures, seems unrelated judging by the time they run?

[Seelengrab]

I don't know what's going on with the failing package jobs, but I think this is ready for review now.

[Seelengrab]

Small bump - would be nice to get some feedback here, whether this is desired for Base etc. I think it could improve modularization of testsuites & runnability of single tests quite a bit, without requiring something heavier.

[JeffBezanson]

Triage approves! Just needs a rebase.

[Seelengrab]

Rebased on current master! Let's see if we can get a green CI run.

[Seelengrab]

Well FreeBSD didn't want to play nice, but everything else passed. I hope this can be merged before it needs another rebase :)

[oscardssmith]

freebsd tests aren't your fault.

[Seelengrab]

Slipped on the Revert button 🤦

[jonas-schulze]

Just noted that this feature allows

@testset "foo" include("foo.jl")

which currently requires begin ... end and is a somewhat common pattern.

[Seelengrab]

Yes, include is not special in that regard - it's treated like any other function. I'm frankly surprised a PR like this hasn't been made sooner 🤷 Maybe I've missed something and this breaks horribly down the road.