patch 9.1.0269: Test for TextChanged is still flaky with ASAN

patch 9.1.0269: Test for TextChanged is still flaky with ASAN

Problem:  Test for TextChanged is still flaky with ASAN.
Solution: Don't index the result of readfile().
          (zeertzjq)

It turns out that with ASAN the file may become empty during a write
even if it's non-empty both before and after the write, in which case
indexing the result of readfile() will error, so use join() instead.

Also don't delete the file halfway the test, just in case it may cause
errors on the next read.

closes: #14421

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>