Add tests for bitvec_write_field()

This function is actively used by OsmoPCU but have not been covered by
tests so far. The test code is based on
Minh-Quang Nguyen <minh-quang.nguyen@nutaq.com> submission with some
modifications.

The test's FIXME will be addressed in follow-up patches.

Change-Id: I2ee544256b8675bc62a42493aab66a8eeee54f90
Related: OS#1526
diff --git a/tests/testsuite.at b/tests/testsuite.at
index 483860f..4a59b22 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -45,6 +45,12 @@
 AT_CHECK([$abs_top_builddir/tests/bits/bitcomp_test], [0], [expout])
 AT_CLEANUP
 
+AT_SETUP([bitfield])
+AT_KEYWORDS([bitfield])
+cat $abs_srcdir/bits/bitfield_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/bits/bitfield_test], [0], [expout])
+AT_CLEANUP
+
 AT_SETUP([conv])
 AT_KEYWORDS([conv])
 cat $abs_srcdir/conv/conv_test.ok > expout