test: add unit tests for Span-parsing helpers
test: add unit tests for Span-parsing helpers

tests the following four functions:
    - Const() [parse constant]
    - Func()  [parse function]
    - Expr()  [parse expression]
    - Split() [split up a string]


Former-commit-id: aa34d0c711bd6bb6d6157b6c804dbf00e36306ad
syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2