The current sample test is incorrect:

test.assert_equals(imperial_units(426, 1), "1ft 4 1/2in")

That's about 16.77 inches, so it should be 1ft 5in.

Sample tests is different from actual tests, which is a bad idea.

The edge cases are also either missing or only exists in sample tests, such as missing ft, in or fractional in, and carrying.

Needs random tests.