tests: Correct tap_generator skip() when count is greater than 1

tests: Correct tap_generator skip() when count is greater than 1

Issue observed
==============

Output when skipping multiple was incorrectly printing the test case number,
eg.

```
ok 3 - Start session `session_ldr8cxix`
41
ok 4 # Skip: Test application 'gen-ust-events-constructor/gen-ust-events-constructor-so' not found
ok 6 # Skip: Test application 'gen-ust-events-constructor/gen-ust-events-constructor-so' not found
ok 8 # Skip: Test application
'gen-ust-events-constructor/gen-ust-events-constructor-so' not found
```

Cause
=====

The `test_number` was adding the current index to the already modified
`self._last_test_case_id`.

Solution
========

Use `self._last_test_case_id` with no changes.

Known drawbacks
===============

None.

Change-Id: I8ff16b83619cf6e6db2636eeccd58725cc03d0f8
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>