🐛 update balance assertion

[Aboudjem]

Updated the balance assertion in the test.

Previously, it was checking that attacker's balance must be higher than 9900 ether, which always pass since the initial balance was 10000.

Now, it checks for a balance greater than 19900 (initial balance 10000 + 9900 from hacking)

[RareSkills]

this is a valid PR, but we'll merge it after the contest to keep things consistent.

[tanim0la]

@RareSkills ok to merge this now?