Add a NUM2BIN test with a 5 byte operand

Summary:
The first operand to OP_NUM2BIN (the number that is to be converted into a binary) is special. It is numeric but can be larger than 4 bytes long and minimal encoding is not required. This revision adds a few tests that confirm this behavior.

• Try to squeeze a 3 byte number into a 2 byte binary vector - should generate IMPOSSIBLE_ENCODING error
• Convert a 5 byte number into a 5 byte binary - should pass
• Convert a 5 byte number into a 6 byte binary - should pass and pad properly
• Convert a 5 byte number into a 4 byte binary - should generate IMPOSSIBLE_ENCODING error
• Convert a 520 byte number into a binary - should pass
• Convert a number that is not minimally encoded - should pass, minimal encoding does not apply to the first operand
• Shrink a number that is not minimally encoded - should pass
• Use non-minimally encoded number for second operand - should fail, must be minimally encoded
• Use 5 byte second operand - should fail with UNKNOWN_ERROR, second operand must be <= 4 bytes long, result too long would generate PUSH_SIZE error

Test Plan: make test

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, deadalnix, danconnolly

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1438