First, some small tips about the testing:
The following are easily shown to be equivalent: a. The Collatz sequence reaches 1 for every starting point x ≥ 1. b. The Collatz sequence reaches a value y < x for ever starting point x ≥ 2. This reduces the work a lot.
You need to make sure that large numbers in a sequence are handled correctly. For example, if you use 64 bit integer arithmetic and don't watch out, you will get integer overflow and therefore rubbish results for relatively small starting values.
Now it's easy if you are a bit careful to write code that tries to test if the Collatz sequence ends at 1, with the following possible outcomes: a. Your code proves the sequence ends. b. Your code notices that it encountered numbers too large for it to handle. c. Your code detects a cycle. d. Your code detects that it didn't reach 1 in a very large number of steps. e. Your code just continues running forever.
Case a is fine. In case b, you change your code. Case c disproves the conjecture. Case e has never been encountered by anyone. Case d has never been encountered by anyone with the right definition of "very large". If you find a number where you can't prove it reaches 1, that's when you need to worry about it - but it's likely it will never happen.