Fix parallel build race condition for test targets

When executing “make test -jX” (with X > 1) to build and run tests in a
build directory, there is a race condition between the
serial_test_low_level target and the test_prep target (a dependency of
serial_test_fishscript and serial_test_interactive).

As far as I can tell, these events happen in a serial build scenario
(“make test” with the “Unix Makefiles” CMake generator):

  1. The fish_tests binary is built and executed.
  2. The test_prep target (a dependency of serial_test_fishscript)
     cleans up test directories.
  3. Tests in test.fish are executed.

In a parallel build scenario, this often happens:

  1. Build of the fish_tests binary is started.
  2. The test_prep target cleans up test directories.
  3. Build of the fish_tests binary is finished.
  4. Execution of the fish_tests binary starts.
  5. Execution of the fish_tests binary finishes.
  6. Tests in test.fish are executed.

However, if building the fish_tests binary is fast enough but not
instant (e.g. when using ccache), this can happen:

  1. Build of the fish_tests binary is started.
  2. Build of the fish_tests binary is finished.
  3. Execution of the fish_tests binary starts.
  4. The test_prep target cleans up test directories.
  5. fish_tests tests that depend on said test directories may,
     depending on timing, fail because they are wiped by test_prep.

Fix this by making test_prep a dependency of serial_test_low_level so
that test_prep can’t interfere with fish_tests execution.

cmake/Tests.cmake

@@ -113,6 +113,7 @@ endforeach(TESTTYPE)
 
 # Now add a dependency chain between the serial versions.
 # This ensures they run in order.
+add_dependencies(serial_test_low_level test_prep)
 add_dependencies(serial_test_fishscript serial_test_low_level)
 add_dependencies(serial_test_interactive serial_test_fishscript)