There are a lot of tests that can be performed. The test file for this model, test_fuel_components.py, currently holds more than 500 lines of code, so I won't be covering it here; however, it will be included in the book's code repository for you to examine at your leisure.
I do want to mention that, quite often (in my experience), testing failures aren't indicative of a problem with the code being tested, but with the tests themselves. For example, look at the following figure:
In this test, two errors are generated. Are they related? Perhaps; a single issue can cause multiple errors, so by fixing and correcting the single issue, multiple errors can be resolved.
In this case, the error being generated in test_gate5_tank2() tells us that the expected value (39085.73879782904) does not equal the real value (19542.86939891452). As noted...