allow to exclude some .purs files from compiling about spago

[f-f]

Spago treats the test folder as the place where to find tests - all the PureScript files in there will be treated as input sources for the test suite of the package.
This means that if you need some fixtures for the tests, as this is the case here, then you shouldn't be including them there. See how in Spago itself we include them in the text-fixtures folder, that is outside of the test tree.

[srghma]

    execArgs:
      - "--exclude-files"
      - "snapshots/*" # TODO: doesnt work
      - "--exclude-files"
      - "test/snapshots/*" # TODO: doesnt work
      - "--exclude-files"
      - "lib/test/snapshots/*" # TODO: doesnt work

doesnt work too