Split up long-running tests for better load balancing (#2662)

* Split up `test/rules/internal_rules.jl`

* Split up `test/tests.jl`

Author: giordano

test/advanced.jl

@@ -0,0 +1,9 @@
+using Enzyme
+using Documenter
+using Test
+
+DocMeta.setdocmeta!(Enzyme, :DocTestSetup, :(using Enzyme); recursive = true)
+
+@testset "DocTests" begin
+    doctest(Enzyme; manual = false)
+end

test/basic.jl

@@ -0,0 +1,8 @@
+using Test
+
+@testset "Threads" begin
+    cmd = `$(Base.julia_cmd()) --threads=1 --startup-file=no threads.jl`
+    @test success(pipeline(cmd, stderr = stderr, stdout = stdout))
+    cmd = `$(Base.julia_cmd()) --threads=2 --startup-file=no threads.jl`
+    @test success(pipeline(cmd, stderr = stderr, stdout = stdout))
+end