Prevent rare rbt_insert_and_remove unit test failures

When two or more absolute, two-label names are added to a completely
empty RBT, an extra, empty node for the root name will be created due to
node splitting.  check_tree() expects that, but the extra node will not
be created when just one name is added to a completely empty RBT.  This
problem could be handled inside check_tree(), but that would introduce
unnecessary complexity into it since adding a single name will result in
a different node count for a completely empty RBT (node count will be 1)
and an RBT containing only an empty node for the root name, created due
to prior node splitting (node count will be 2).  Thus, first explicitly
create a node for the root name to prevent rare check_tree() failures
caused by a single name being added in the first iteration of the
insert/remove loop.
25 jobs for 471-fix-rbt_insert_and_remove-unit-test in 10 minutes and 35 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #36449
docker linux amd64

00:00:13

 
  Build
passed build:centos:centos6:amd64 #36450
amd64 docker linux

00:03:14

passed build:centos:centos7:amd64 #36451
amd64 docker linux

00:04:09

passed build:clang:debian:stretch:amd64 #36452
amd64 docker linux

00:02:27

passed build:clang:debian:stretch:i386 #36456
i386 docker linux

00:04:48

passed build:debian:jessie:amd64 #36453
amd64 docker linux

00:04:13

passed build:debian:sid:amd64 #36455
amd64 docker linux

00:04:38

passed build:debian:sid:i386 #36457
i386 docker linux

00:02:24

passed build:debian:stretch:amd64 #36454
amd64 docker linux

00:04:27

passed pkcs11:build:debian:sid:amd64 #36458
amd64 docker linux

00:04:41

 
  Test
passed pkcs11:systemtest:debian:sid:amd64 #36473
amd64 docker linux

00:04:48

passed pkcs11:unittest:debian:sid:amd64 #36472
amd64 docker linux

00:00:52

passed systemtest:centos:centos6:amd64 #36466
amd64 docker linux

00:04:54

passed systemtest:centos:centos7:amd64 #36467
amd64 docker linux

00:04:53

passed systemtest:debian:jessie:amd64 #36468
amd64 docker linux

00:04:50

passed systemtest:debian:sid:amd64 #36470
amd64 docker linux

00:04:45

passed systemtest:debian:sid:i386 #36471
i386 docker linux

00:05:03

passed systemtest:debian:stretch:amd64 #36469
amd64 docker linux

00:04:55

passed unittest:centos:centos6:amd64 #36459
amd64 docker linux

00:00:55

passed unittest:centos:centos7:amd64 #36460
amd64 docker linux

00:00:55

passed unittest:clang:debian:stretch:amd64 #36464
amd64 docker linux

00:00:49

passed unittest:debian:jessie:amd64 #36461
amd64 docker linux

00:00:55

passed unittest:debian:sid:amd64 #36463
amd64 docker linux

00:00:52

passed unittest:debian:sid:i386 #36465
i386 docker linux

00:00:52

passed unittest:debian:stretch:amd64 #36462
amd64 docker linux

00:00:50