test/inccommand_spec: avoid indeterminism

- Use shell-test.c to avoid the squishiness of system shells.
- Use screen:expect_unchanged() to avoid hardcoded (brittle) test.

Fails correctly if 5020daa6e5ce is reverted (remove terminal_check(),
restore redraw() in refresh_timer_cb()):

    [  ERROR   ] test/functional/ui/inccommand_spec.lua @ 2550: :substitute with inccommand during :terminal activity
    test/functional/helpers.lua:402:
    retry() attempts: 2
    test/functional/ui/screen.lua:579: Row 8 did not match.
    Expected:
      |foo bar baz                   |
      |bar baz fox                   |
      |bar foo baz                   |
      |{15:~                             }|
      |{15:~                             }|
      |{15:~                             }|
      |{11:[No Name] [+]                 }|
      |*26: xxx                       |
      |27: xxx                       |
      |28: xxx                       |
      |29: xxx                       |
      |30: xxx                       |
      |                              |
      |{10:term                          }|
      |:%s/foo/ZZZ^                   |
    Actual:
      |foo bar baz                   |
      |bar baz fox                   |
      |bar foo baz                   |
      |{15:~                             }|
      |{15:~                             }|
      |{15:~                             }|
      |{11:[No Name] [+]                 }|
      |*107: xxx                      |
      |108: xxx                      |
      |109: xxx                      |
      |110: xxx                      |
      |111: xxx                      |
      |                              |
      |{10:term                          }|
      |:%s/foo/ZZZ^                   |

0 files changed, 0 insertions, 0 deletions