expose testRun name to be configureable in gradle

[Danny Preussler]

currently only ProjectName can be configured from outside:

testdroid {
projectName 'My project'
...

I would also like the configure the testRun name (called "variantName" in the uploadApk() method here)

To something like:

testdroid {
projectName 'My project'
testRunName 'automated build'
...

or so would be great