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.err b/tests/alloc/AllocTest.err
index 23eea09..db95601 100644
--- a/tests/alloc/AllocTest.err
+++ b/tests/alloc/AllocTest.err
@@ -1,5 +1,7 @@
 No TFI available.
 - Failed allocating TS=2, no USF available
+No TFI available.
+- Failed allocating TS=1, no USF available
 - Failed allocating TS=3, no USF available
 No suitable uplink slots available
 No suitable uplink slots available