Add note about include (a hacky solution for now)

Author: gaurav-arya

test/runtests.jl

@@ -541,6 +541,8 @@ end
 end
 
 @testset "AbstractFFTs upstream tests" begin
+    # A necessary hack since AbstractFFTsTestUtils is not its own registered package yet.
+    # See https://github.com/JuliaMath/AbstractFFTs.jl/pull/78
     include(joinpath(pathof(AbstractFFTs), "..", "..", "test", "testbackend.jl"))
     using .AbstractFFTsTestUtils
     test_fft_backend()