Implement isdiag(::Bidiagonal), add tests (#32694)

(cherry picked from commit 25eb8c6)

Author: garrison

stdlib/LinearAlgebra/src/bidiag.jl

@@ -246,6 +246,7 @@ end
 
 istriu(M::Bidiagonal) = M.uplo == 'U' || iszero(M.ev)
 istril(M::Bidiagonal) = M.uplo == 'L' || iszero(M.ev)
+isdiag(M::Bidiagonal) = iszero(M.ev)
 
 function tril!(M::Bidiagonal, k::Integer=0)
     n = length(M.dv)

stdlib/LinearAlgebra/test/bidiag.jl

@@ -150,6 +150,10 @@ Random.seed!(1)
             @test triu!(bidiagcopy(dv,ev,:U))    == Bidiagonal(dv,ev,:U)
             @test_throws ArgumentError triu!(bidiagcopy(dv, ev, :U), -n)
             @test_throws ArgumentError triu!(bidiagcopy(dv, ev, :U), n + 2)
+            @test !isdiag(Bidiagonal(dv,ev,:U))
+            @test !isdiag(Bidiagonal(dv,ev,:L))
+            @test isdiag(Bidiagonal(dv,zerosev,:U))
+            @test isdiag(Bidiagonal(dv,zerosev,:L))
         end
 
         Tfull = Array(T)