Normally, one rolls Steel against Hesitation, which is pretty inevitably a challenging test for a lot of characters.
Of course, to advance Steel, some routine and difficult tests are also required.
Are situational tests the only way to get those?
You should run through the Situational Steel tests list at the end of every session. Also, don’t forget Steel Test Advantages and Disadvantages on page 363. Even if your Steel is B7 and your Hesitation is 5, you’ll get a challenging Steel test when confronted with the zombies (Seeing the living dead, +4 ob).