Basic tests for the file-watching mode.

----------------------------------------------------------------------------------
* Compile a simple rule

  $ echo "(lang dune 2.0)" > dune-project

  $ cat > x <<EOF
  > original-contents
  > EOF

  $ cat >dune <<EOF
  > (rule
  >  (target y)
  >  (deps x)
  >  (action (system "cat x > y")))
  > EOF

  $ start_dune

  $ build y
  Success
  $ cat _build/default/y
  original-contents

  $ echo new-contents > x

  $ build y
  Success
  $ cat _build/default/y
  new-contents

  $ echo new-contents2 > x

  $ build y
  Success
  $ cat _build/default/y
  new-contents2

----------------------------------------------------------------------------------
* File rename

  $ mv x z
  $ build y
  Failure

  $ echo new-contents3 > z

  $ build y
  Failure

  $ mv z x
  $ build y
  Success
  $ cat _build/default/y
  new-contents3

  $ with_timeout dune shutdown
  $ cat .#dune-output
  Success, waiting for filesystem changes...
  Success, waiting for filesystem changes...
  Success, waiting for filesystem changes...
  File "dune", lines 1-4, characters 0-59:
  1 | (rule
  2 |  (target y)
  3 |  (deps x)
  4 |  (action (system "cat x > y")))
  Error: No rule found for x
  Had 1 error, waiting for filesystem changes...
  File "dune", lines 1-4, characters 0-59:
  1 | (rule
  2 |  (target y)
  3 |  (deps x)
  4 |  (action (system "cat x > y")))
  Error: No rule found for x
  Had 1 error, waiting for filesystem changes...
  Success, waiting for filesystem changes...
