The hidden tests do not always test for numbers like "7000" or "400". Code, that does not handle those numbers, can pass the hidden tests if the user is lucky. I suggest to hardcode a number like "7000" or "400" into the hidden test.

• In modern C, the return type should be a bool (from <stdbool.h>)
• the input type should be marked as unsigned
• you need to put the fixed tests in the full tests suite as well
• suggested edge cases that should be in the fixed tests: 0, 10, 1000, 1001, 2001
• the functions defined in the tests must all be static, otherwise the user might call them
• you should extract your reference solution to a function to make the random tests more maintainable

is_decimal or is_integer?