Support scanning tests in src directories

[tengstrand]

People sometimes put test code together with the src code. Today we don't run those tests. It would be good if this could be configurable.

To turn it on globally, we could put an :include-src-dirs flag in workspace.edn at the root to affect all tests:

{
 ...
 :test {:include-src-dirs true}
}

or per project:

{
 ...
 :projects {"myproject" {...
                         :test {:include-src-dirs true}}
}

or even per brick:

{
 ...
 :bricks {"mybrick" {...
                     :test {:include-src-dirs true}}
}