Minor tick fix [backport to 1.4.x]

This pull request ensures minor ticks are positioned correctly with `AutoMinorLocator`, fixing the bug described in issue #3557.
The problem is that this test is creating a figure which is leaking out and getting re-used in the next test (global state strikes again!).

This test either needs an `@cleanup` decorator or to be re-written to test the locator without creating a figure/axes.  I strongly prefer the second.