patch 9.2.0429: tests: flaky screendump Test_smoothscroll_incsearch()

patch 9.2.0429: tests: flaky screendump Test_smoothscroll_incsearch()

Problem:  tests: flaky screendump Test_smoothscroll_incsearch()
Solution: Replace screendump test by WaitForAssert()
          (Yasuhiro Matsumoto)

VerifyScreenDump fails consistently on the macos-15-intel CI runner.
Replace the dump comparisons with assertions that verify the actual
invariant under test: that the visible buffer view stays unchanged
across the four incremental-search keystrokes (i.e. skipcol is not
reset). Drop the now-unused dump files.

closes: #20118

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>