freebsd: Avoid /bin/true in commandtest

Rely on PATH and use just true, because on FreeBSD it's /usr/bin/true.

tests/commanddata/test16.log

-A=B /bin/true C
+A=B true C

tests/commandtest.c

@@ -566,9 +566,9 @@ cleanup:
  */
 static int test16(const void *unused ATTRIBUTE_UNUSED)
 {
-    virCommandPtr cmd = virCommandNew("/bin/true");
+    virCommandPtr cmd = virCommandNew("true");
     char *outactual = NULL;
-    const char *outexpect = "A=B /bin/true C";
+    const char *outexpect = "A=B true C";
     int ret = -1;
     int fd = -1;
 
@@ -610,7 +610,7 @@ cleanup:
  */
 static int test17(const void *unused ATTRIBUTE_UNUSED)
 {
-    virCommandPtr cmd = virCommandNew("/bin/true");
+    virCommandPtr cmd = virCommandNew("true");
     int ret = -1;
     char *outbuf;
     char *errbuf;