alloc: Use least reserved PDCH for algo A

Currently the slot selection of algorithm A is based on the current
slot usage by active TBF. Especially in the Dl after UL case which
reflects the commen use case "MS initiates TCP connection", the
resulting distribution is not optimal with respect to PDCH usage.

This commit changes the implementation to use the slot reservation
information instead.

Sponsored-by: On-Waves ehf
diff --git a/tests/alloc/AllocTest.err b/tests/alloc/AllocTest.err
index 7bb2995..af9b134 100644
--- a/tests/alloc/AllocTest.err
+++ b/tests/alloc/AllocTest.err
@@ -11,7 +11,7 @@
 No TFI available.
 No TFI available.
 No TFI available.
-- Failed to allocate a TS, no USF available
+No TFI available.
 No TFI available.
 No TFI available.
 No TFI available.