tests/qemu-iotests/testrunner: Supply a test plan in TAP mode

Quoting the TAP specification: "The plan tells how many tests will be
run [...]. It’s a check that the test file hasn’t stopped prematurely."
That's a good idea of course, so let's support that in the iotest
testrunner, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220223095816.2663005-1-thuth@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

tests/qemu-iotests/testrunner.py

@@ -388,6 +388,7 @@ class TestRunner(ContextManager['TestRunner']):
 
         if self.tap:
             self.env.print_env('# ')
+            print('1..%d' % len(tests))
         else:
             self.env.print_env()