Fix bad usage string assumption in test

$0 is not changed by functions so this could not have worked (strange it
*did* work on Fedora, I still have to find out why).

tests/cli/TF_RUN

@@ -19,6 +19,7 @@ tf_do_subtest() {
     SATURNIN_CACHE_HOME=chm
     SATURNIN_LIBEXEC=lex
     SATURNIN_LIBEXEC_PREFIX=lexpfx-
+    PRETTY_USAGE=self=lexpfx
     mkdir lex
     touch lex/lexpfx-{foo,bar,baz}
     chmod +x lex/lexpfx-{foo,bar,baz}

tests/cli/oracle/usage.stderr

@@ -1,5 +1,5 @@
 usage:
-  saturnin__usage [options] COMMAND [ARG...]
+  lexpfx [options] COMMAND [ARG...]
 
 options:
   -D, --full-debug   turn on gory debugging