Create a cram test and try to run it with DUNE_BUILD_DIR set to an absolute
path

  $ make_dune_project 3.5

  $ cat >foo.t <<EOF
  >   $ echo "  $ echo bar" >bar.t
  >   $ dune runtest
  > EOF

  $ DUNE_BUILD_DIR=$PWD/tmp dune runtest --auto-promote
  File "foo.t", line 1, characters 0-0:
  --- foo.t
  +++ foo.t.corrected
  @@ -1,2 +1,4 @@
     $ echo "  $ echo bar" >bar.t
     $ dune runtest
  +  Error: Another Dune instance is currently running. Aborting...
  +  [1]
  Promoting
    $TESTCASE_ROOT/tmp/default/foo.t.corrected
    to foo.t.
  [1]
  $ cat foo.t
    $ echo "  $ echo bar" >bar.t
    $ dune runtest
    Error: Another Dune instance is currently running. Aborting...
    [1]
