alloc/test: Enhance test_alloc_a

This commit adds:
 - an assertion to check that count is valid,
 - an assertion the verify that tbf_alloc did not fail
 - a slots parameter to specify the enabled slots
 - additional test invocations with more slots enabled

Sponsored-by: On-Waves ehf
diff --git a/tests/alloc/AllocTest.ok b/tests/alloc/AllocTest.ok
index f5df0f8..89393fc 100644
--- a/tests/alloc/AllocTest.ok
+++ b/tests/alloc/AllocTest.ok
@@ -1,5 +1,7 @@
 Testing alloc_a direction(0)
 Testing alloc_a direction(1)
+Testing alloc_a direction(0)
+Testing alloc_a direction(1)
 Going to test multislot assignment MS_CLASS=0
 Testing UL then DL assignment.
 PDCH[6] is used for UL