Change the test infrastructure so that tests are easier to maintain and

Change the test infrastructure so that tests are easier to maintain and
so that make(1) will run in an almost clean environment and enhance the
description of the test infrastructure.

Add the ability to have multiple tests carried out per test script.

Give some tests more meaningful names.

Fix the usage message from the test scripts.

Make it possible to pass several commands to the test scripts like:
'sh test.t setup run compare clean'.