Use SciMLTesting v1.2 folder-based run_tests (#44)

Convert the test suite to the SciMLTesting v1.2 folder-discovery model:

- runtests.jl becomes `using SciMLTesting; run_tests()`.
- Core: the monolithic core.jl is split into self-contained top-level files
  (unit_tests.jl, end_to_end_tests.jl, issue_3.jl, commutation_tests.jl), each
  run isolated in its own @safetestset by folder discovery. The previously
  unconditional "Invalid usage error path" testset becomes a self-contained
  Core file (invalid_usage_tests.jl) — it is a macro-correctness unit test that
  belongs to Core; CI still exercises it on the Core lane.
- QA stays at test/qa/ with its existing qa.jl + Project.toml sub-env.
- SciMLTesting + SafeTestsets added to the main test env and the QA sub-env;
  Pkg dropped from the test env (no test file uses it; the harness owns Pkg
  ops). test_groups.toml unchanged.
- Removed the checked-in test/Manifest.toml: it is gitignored and was stale
  after the dependency change, so Pkg re-resolves it fresh.

Core's assertion set is preserved exactly (42 @test/@test_throws before and
after, including the moved invalid-usage check). Per the v1.2 folder semantics
the QA group is its own CI lane and is not part of the local `All` default.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

test/Manifest.toml

@@ -1,8 +1,12 @@
 [deps]
-Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
+SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
+SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283"
 Suppressor = "fd094767-a336-5f1f-9728-57cf17d0bbfb"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [compat]
+SafeTestsets = "0.0.1, 0.1"
+SciMLTesting = "1"
 Suppressor = "0.2.8"
+Test = "1"

test/Project.toml

@@ -0,0 +1,33 @@
+using ConcreteStructs
+using Test
+
+# this has been working for a while already
+Base.@kwdef @concrete struct Foo2{T <: Real}
+    x::T = 1.0
+    y = true
+    z = 42
+end
+
+foo = Foo2()
+@test typeof(foo) === Foo2{Float64, Bool, Int64}
+
+# this is new
+@concrete Base.@kwdef struct Foo3{T <: Real}
+    x::T = 1.0
+    y = true
+    z = 42
+end
+
+foo = Foo3()
+@test typeof(foo) === Foo3{Float64, Bool, Int64}
+
+foo = Foo3(z = [1, 2, 3])
+@test typeof(foo) === Foo3{Float64, Bool, Vector{Int64}}
+
+# terse test case, unfortunately doesn't work the other way around. But also never did.
+@concrete terse Base.@kwdef struct TerseKWDef
+    a = 1.0
+    b = true
+end
+terse_kw_def = TerseKWDef(b = false)
+@test typeof(terse_kw_def) === TerseKWDef{Float64, Bool}