build: rely on configure to select which unit tests to build

There's no reason to check the OS to decide whether to build some unit
tests if we can rely on the configure parameters.  For instance, on
FreeBSD, RDMA unit tests can be built and run successfully, while vhost
is always disabled in configure.

Fixes #2941.

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/17182

 (master)

(cherry picked from commit 1b49dcef)
Change-Id: I0073783dcafa932f3a7f48fa553e04be7ae44f84
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/17540


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>