Use SciMLTesting v1.0.0 (run_tests harness) (#158)

Replace the hand-written GROUP-dispatch in test/runtests.jl with a single
SciMLTesting.run_tests call. Behavior-equivalent refactor:

- core = the existing Core test body (verbatim, re-indented by Runic).
- QA is registered as a groups["QA"] entry with a declared sub-env
  (test/qa), so it runs only for GROUP="QA" and is excluded from "All" --
  matching the original `if GROUP == "QA"` dispatch (QA never ran under All).
- qa/qa.jl is included verbatim (custom Aqua/JET bodies preserved, no
  collapse into run_qa, so assertions are unchanged).

Project.toml: add SciMLTesting (UUID 09d9d899-5365-40a9-917a-5f67fddea283)
to [extras]/[targets].test/[compat]; drop Pkg (only used by the old
harness; run_tests bundles Pkg internally).

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

Author: 3 people

Project.toml

@@ -28,13 +28,13 @@ SymbolicLimits = "0.2.4, 1.1"
 SymbolicUtils = "4.20.1"
 Symbolics = "7.12.0"
 TermInterface = "2"
-Pkg = "1"
+SciMLTesting = "1"
 Test = "1"
 julia = "1.10"
 
 [extras]
-Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
+SciMLTesting = "09d9d899-5365-40a9-917a-5f67fddea283"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["Pkg", "Test"]
+test = ["SciMLTesting", "Test"]