Add assertSelectedLabel and verifySelectedLabel

[Plotist]

By request from #160

[Plotist]

@corevo is there something that needs to be done for it to be merged ?

[corevo]

No, this is on me, I want to manually check this, also applies for #171
I can just think of a few edge cases that may arise and I want to make sure that the automatic suite covers them, since the side format not as human readable as code, I'll have to check this out in the IDE.

[corevo]

Since behind the scenes when we export verify and assert are the same, the test is needlessly slower.
Instead change the selected label, and check that it works.