ahirreddy and I found some cases where assume and assert were mixed up in test suites.
If an assume call fails then it will cause the test to be marked as skipped instead of failed: this is often used to skip tests if certain prerequisites are missing (e.g. to disable certain tests on Windows). If assume is mistakenly used in place of assert then this can cause logically-failing tests to be skipped, masking bugs.
This patch fixes several such cases, replacing certain assume calls with assert.