patch 9.1.2050: tests: Test_cd_completion may fail

patch 9.1.2050: tests: Test_cd_completion may fail

Problem:  tests: Test_cd_completion() may fail depending on the contents
          of the root directory of the current drive on Windows.
          readdir() may return a directory that cannot "cd" to, causing
          this test to fail. An example of such a directory is
          "System Volume Information" which only admin can "cd" to.
Solution: When determining the directory to use for testing, use the
          directory that we actually "cd" to successfully.
          In addition, directories with '$' in their names are also
          excluded, as they are considered environment variables during
          completion and do not work as expected.
          Example: "$RECYCLE.BIN" (Muraoka Taro).

closes: #19078

Signed-off-by: Muraoka Taro <koron.kaoriya@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>