Ensure positive-definite matrix in lapack posv test

[jishnub]

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.

[ViralBShah]

@KristofferC - Seeing this in the i686 CI. Is this a problem?

- PackageCompiler: compiling nonincremental system image
  | ERROR: IOError: could not spawn `/shells/bash -c '(pwd) && true'`: no such file or directory (ENOENT)
  | Running parallel tests with:
  | getpid() = 450
  | nworkers() = 16
  | nthreads(:interactive) = 1
  | nthreads(:default) = 1
  | Sys.CPU_THREADS = 16
  | Sys.total_memory() = 503.719 GiB
  | Sys.free_memory() = 470.665 GiB
  |  

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.96%. Comparing base (4c9e125) to head (545d307).
Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1238   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files          34       34           
  Lines       15475    15475           
=======================================
  Hits        14231    14231           
  Misses       1244     1244           

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