bitvec: Test and fix regression for C++->C conversion

bitvec_read_field/bitvec_write_field in the PCU used a C++ reference
and when porting to C it was decided to pass the parameter by value
and this lost the "back propagation" of the new index. Change the
parameter to be an in/out parameter and this way do not have a silent
semantic break in the osmo-pcu (where we copy the reference in csn.1
by value) and have a true compile failure.

Add Max's simple test for bitvec_unhex function leaving the checking
of bitvec_read_field and the side effect in the datastructure about
the number of bits still open.
diff --git a/tests/bitvec/bitvec_test.ok b/tests/bitvec/bitvec_test.ok
index 1f329af..8d944bc 100644
--- a/tests/bitvec/bitvec_test.ok
+++ b/tests/bitvec/bitvec_test.ok
@@ -1,2 +1,17 @@
 === start test_byte_ops ===
 === end test_byte_ops ===
+1 -=>
+48282407a6a074227201000b2b2b2b2b2b2b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+48282407a6a074227201000b2b2b2b2b2b2b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+1 -=>
+47240c00400000000000000079eb2ac9402b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+47240c00400000000000000079eb2ac9402b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+1 -=>
+47283c367513ba333004242b2b2b2b2b2b2b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+47283c367513ba333004242b2b2b2b2b2b2b2b2b2b2b2b0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+1 -=>
+deadface000000000000000000000000000000beeffeed0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+deadface000000000000000000000000000000beeffeed0000000000000000000000000000000000000000000000000000000000000000000000000000000000
+0 -=>
+fffffaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+fffffaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa