Fix "no method matching" test (#288)

The text of the MethodError message is not a stable API, so use the
currently stable API of checking the field value. Discussion at
JuliaLang/julia#47369.

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: apaz-cli

test/higherorderfns.jl

@@ -670,7 +670,7 @@ using SparseArrays.HigherOrderFns: SparseVecStyle, SparseMatStyle
     end
     @test err isa MethodError
     @test !occursin("is ambiguous", sprint(showerror, err))
-    @test occursin("no method matching _copy(::typeof(rand))", sprint(showerror, err))
+    @test err.f === SparseArrays.HigherOrderFns._copy
 end
 
 @testset "Sparse outer product, for type $T and vector $op" for