armstrong-numbers: Include excluded tests

[tasxatzial]

Added the final two tests involving big integers. Without these tests, some solutions improperly relied on Java interop for exponentiation, which is incorrect since Java's exponentiation functions return floating-point numbers instead of integers, as required here. Additionally, without these tests, incorrect solutions using Java interop could pass all tests. This PR addresses both issues.

Also

• Updated the armstrong_numbers.clj starter file with a proper function template
• Explicitly check with true? and false? as true and false are the exact values expected to be returned by armstrong?
• Updated example solution to pass the new tests