Add option to run spt tests

There does not seem to be an option to run spt tests

### I tried to do this:

Create a spt file.

### I expected this to happen:

File is syntax highlighted and right-clicking in the tree-view should have an option to run it.

### Instead, this happened:

spt test files are not recognized and I am unable to run them.

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> Spoofax: 2.1.0-SNAPSHOT
