tests: add unit tests for OP_CAT

This commit introduces unit tests for opcat in script_tests.json.
Additionally, it creates new test utilities for future Taproot script
tests within script_tests.json. The key features of this commit are the
addition of three new tags: `#SCRIPT#`, `#CONTROLBLOCK#`, and
`#TAPROOTOUTPUT#`. These tags streamline the test creation process by
eliminating the need to manually generate these components outside the
test suite.

* `#SCRIPT#`: Parses Tapscript and outputs a byte string of opcodes.
* `#CONTROLBLOCK#`: Automatically generates the control block for a given
Taproot output.
* `#TAPROOTOUTPUT#`: Generates the final Taproot scriptPubKey.