[backport#16887 3/3] test: add unit tests for Span-parsing helpers

Summary:
tests the following four functions:

• Const() [parse constant]
• Func() [parse function]
• Expr() [parse expression]
• Split() [split up a string]

https://github.com/bitcoin/bitcoin/pull/16887/commits/bb36372b8f2bd675313ae8553ceb61f28c2c1afd

Depends on D6878

Concludes backport of Core PR16887

Test Plan:

ninja check-bitcoin-util_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6879