Do not rely on command prefixes in tests

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

Author: rgrinberg

test/blackbox-tests/test-cases/findlib-dynload.t/run.t

@@ -51,17 +51,17 @@
   b: called
   a: called
 
-  $ dune exe mytool_auto
-  m: init
-  a: init
-  b: init
-  b: registering
-  b: called
-  a: called
+  $ dune exec mytool_auto
+  dune: unknown command `exe'.
+  Usage: dune COMMAND ...
+  Try `dune --help' for more information.
+  [1]
 
-  $ dune exe mytool c_thread
-  m: init
-  c_thread: registering
+  $ dune exec mytool c_thread
+  dune: unknown command `exe'.
+  Usage: dune COMMAND ...
+  Try `dune --help' for more information.
+  [1]
 
   $ cat _build/default/.main.eobjs/findlib_initl.ml-gen
   Findlib.record_package Findlib.Record_core "mytool";;

test/blackbox-tests/test-cases/format-dune-file.t/run.t

@@ -162,7 +162,8 @@ Files in OCaml syntax are copied verbatim (but error when passed in stdin).
 
 Non 0 error code:
 
-  $ echo "(" | dune format ; echo $?
-  File "", line 2, characters 0-0:
-  Error: unclosed parenthesis at end of input
+  $ echo "(" | dune format-dune-file ; echo $?
+  dune: unknown command `format'.
+  Usage: dune COMMAND ...
+  Try `dune --help' for more information.
   1

test/blackbox-tests/test-cases/pipe-actions.t/run.t

@@ -52,15 +52,11 @@ The makefile version of pipe actions uses actual pipes:
   >    (pipe-outputs (run a) (run b) (run c)))))
   > EOF
 
-  $ dune rule -m target
-  _build/default/target: _build/install/default/bin/a \
-    _build/install/default/bin/b _build/install/default/bin/c
-  	mkdir -p _build/default; \
-  	mkdir -p _build/default; \
-  	cd _build/default; \
-  	../install/default/bin/a  2>&1 |  \
-  	  ../install/default/bin/b | ../install/default/bin/c  &> target
-  
+  $ dune rules -m target
+  dune: unknown command `rule'.
+  Usage: dune COMMAND ...
+  Try `dune --help' for more information.
+  [1]
   $ cat >dune <<EOF
   > (executable
   >  (public_name apl) (name append_to_line) (modules append_to_line))