iotests: Drop readlink -f

On macOS, (out of the box) readlink does not have -f.  We do not really
need readlink here, though, it was just a replacement for realpath
(which is not available on our BSD test systems), which we needed to
make the $(dirname) into an absolute path.

Instead of using either, just use "cd; pwd" like is done for
$source_iotests.

       ("iotests: Allow running from different directory")

Fixes: b1cbc33a3971b6bb005d5ac3569feae35a71de0f
Reported-by: Claudio Fontana <cfontana@suse.de>
Reported-by: Thomas Huth <thuth@redhat.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200914145606.94620-1-mreitz@redhat.com>
Message-Id: <20200915134317.11110-3-alex.bennee@linaro.org>

tests/qemu-iotests/check

@@ -44,7 +44,7 @@ then
         _init_error "failed to obtain source tree name from check symlink"
     fi
     source_iotests=$(cd "$source_iotests"; pwd) || _init_error "failed to enter source tree"
-    build_iotests=$(readlink -f $(dirname "$0"))
+    build_iotests=$(cd "$(dirname "$0")"; pwd)
 else
     # called from the source tree
     source_iotests=$PWD