Fix CI (#1992)

Author: devmotion

test/essential/ad.jl

@@ -31,13 +31,21 @@
         x = map(x->Float64(x), vi[SampleFromPrior()])
 
         trackerℓ = LogDensityProblemsAD.ADgradient(TrackerAD(), ℓ)
-        @test trackerℓ isa LogDensityProblemsAD.LogDensityProblemsADTrackerExt.TrackerGradientLogDensity
+        if isdefined(Base, :get_extension)
+            @test trackerℓ isa Base.get_extension(LogDensityProblemsAD, :LogDensityProblemsADTrackerExt).TrackerGradientLogDensity
+        else
+            @test trackerℓ isa LogDensityProblemsAD.LogDensityProblemsADTrackerExt.TrackerGradientLogDensity
+        end
         @test trackerℓ.ℓ === ℓ
         ∇E1 = LogDensityProblems.logdensity_and_gradient(trackerℓ, x)[2]
         @test sort(∇E1) ≈ grad_FWAD atol=1e-9
 
         zygoteℓ = LogDensityProblemsAD.ADgradient(ZygoteAD(), ℓ)
-        @test zygoteℓ isa LogDensityProblemsAD.LogDensityProblemsADZygoteExt.ZygoteGradientLogDensity
+        if isdefined(Base, :get_extension)
+            @test zygoteℓ isa Base.get_extension(LogDensityProblemsAD, :LogDensityProblemsADZygoteExt).ZygoteGradientLogDensity
+        else
+            @test zygoteℓ isa LogDensityProblemsAD.LogDensityProblemsADZygoteExt.ZygoteGradientLogDensity
+        end
         @test zygoteℓ.ℓ === ℓ
         ∇E2 = LogDensityProblems.logdensity_and_gradient(zygoteℓ, x)[2]
         @test sort(∇E2) ≈ grad_FWAD atol=1e-9