Fix CI: Exclude Enzyme tests from Julia pre

[ChrisRackauckas-Claude]

Summary

This PR fixes CI by splitting tests into separate groups and excluding Enzyme tests from Julia pre-release versions.

The Julia 1.13-alpha tests fail due to an LLVM data layout incompatibility in Enzyme:

LLVM error: Added modules have incompatible data layouts: 
e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128 (module) vs 
e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128-ni:10:11:12:13 (jit)

This is an upstream Enzyme issue tracked at EnzymeAD/Enzyme.jl#2699.

Changes

1. Split tests into groups: Modified test/runtests.jl to support Core and Enzyme test groups via the GROUP environment variable
2. Matrix exclude: Updated Tests.yml to use matrix exclude pattern (similar to OrdinaryDiffEq) to skip Enzyme tests on Julia pre-release

Test Matrix

Julia Version	Core	Enzyme
1 (stable)	✅	✅
lts	✅	✅
pre	✅	❌ (excluded)

Test Plan

• Core tests pass locally
• Enzyme tests pass locally
• CI should pass for all included matrix combinations

cc @ChrisRackauckas

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Added fix for formatting issue in test/runtests.jl - missing spaces around the ≈ operator.

cc @ChrisRackauckas