Ensure positive-definite matrix in lapack posv test (#1238)

The test failure in
https://buildkite.com/julialang/linearalgebra-dot-jl/builds/330#01954c88-5de5-42fb-a0b4-e10f487d934e
is because the matrix is not positive-definite. Since we construct the
matrix by ourselves, we may ensure that the eigenvalues are all positive
by defining it as ~~`Q*D*Q'` for an orthogonal `Q`~~ `A'A + I`. This way
we avoid such floating-point issues.

Author: ViralBShah

test/lapack.jl

@@ -687,7 +687,7 @@ end
     @testset for elty in (Float32, Float64, ComplexF32, ComplexF64)
         local n = 10
         a = randn(elty, n, n)
-        A = a'*a
+        A = a'*a + I
         B = rand(elty, n, n)
         D = copy(A)
         C = copy(B)