Add inference tests

Author: jishnub

stdlib/LinearAlgebra/test/bidiag.jl

@@ -846,7 +846,7 @@ end
 
         s = SizedArrays.SizedArray{(2,2)}([1 2; 3 4])
         B = Bidiagonal(fill(s,4), fill(s,3), :U)
-        @test B[2,1] isa typeof(s)
+        @test @inferred(B[2,1]) isa typeof(s)
         @test all(iszero, B[2,1])
     end
 end

stdlib/LinearAlgebra/test/diagonal.jl

@@ -822,7 +822,7 @@ end
 
         s = SizedArrays.SizedArray{(2,2)}([1 2; 3 4])
         D = Diagonal(fill(s,3))
-        @test D[1,2] isa typeof(s)
+        @test @inferred(D[1,2]) isa typeof(s)
         @test all(iszero, D[1,2])
     end
 end