relax a test in test/rational.jl

As noted by simeonschaub on the original issue #41489, this only used
to work by accident anyway.

Author: nsajko

test/rational.jl

@@ -803,7 +803,7 @@ end
     @test Core.Compiler.return_type(-, NTuple{2, Rational}) == Rational
 
     A=Rational[1 1 1; 2 2 2; 3 3 3]
-    @test @inferred(A*A) isa Matrix{Rational}
+    @test @inferred(A*A) isa Matrix
 end
 
 @testset "issue #42560" begin