bpo-35189: Fix eintr_tester.py (GH-10637)

Call setitimer() before each test method, instead of once per test
case, to ensure that signals are sent in each test method.
Previously, only the first method of a testcase class got signals.

Changes:

* Replace setUpClass() with setUp() and replace tearDownClass() with
  tearDown().
* tearDown() now ensures that at least one signal has been sent.
* Replace support.run_unittest() with unittest.main() which has
  a nicer CLI and automatically discover test cases.