src/file.bash: Align code of `assert_not_file_owner` to `assert_file_owner`

Change the code of `assert_not_file_owner` to the version of
`assert_file_owner` introduced in PR #38.

The removal of the stray sudo calls fixes issue #49.

src/file.bash

diff --git a/src/file.bash b/src/file.bash
index e96c3c2..2fce974 100644
--- a/src/file.bash
+++ b/src/file.bash
@@ -818,27 +818,20 @@
 assert_not_file_owner() {
   local -r owner="$1"
   local -r file="$2"
-  if [[ `uname` == "Darwin" ]]; then
-  sudo chown root ${TEST_FIXTURE_ROOT}/dir/owner
-  sudo chown daemon ${TEST_FIXTURE_ROOT}/dir/notowner
-  if [ `stat -f '%Su' "$file"` = "$owner" ]; then
+  if [[ "$(uname)" == "Darwin" ]]; then
+    __cmd_param="-f %Su"
+  elif [[ "$(uname)" == "Linux" ]]; then
+    __cmd_param="-c %U"
+  fi
+  __o=$(stat $__cmd_param "$file")
+
+  if [[ "$__o" == "$owner" ]]; then
     local -r rem="${BATSLIB_FILE_PATH_REM-}"
     local -r add="${BATSLIB_FILE_PATH_ADD-}"
     batslib_print_kv_single 4 'path' "${file/$rem/$add}" \
-      | batslib_decorate "given user is the $owner, but it was expected not to be" \
+      | batslib_decorate "user $owner is the owner of the file, but it was expected not to be" \
       | fail
   fi
-  elif [[ `uname` == "Linux" ]]; then
-  sudo chown root ${TEST_FIXTURE_ROOT}/dir/owner
-  sudo chown daemon ${TEST_FIXTURE_ROOT}/dir/notowner
-    if [ `stat -c "%U" "$file"` = "$owner" ]; then
-    local -r rem="${BATSLIB_FILE_PATH_REM-}"
-    local -r add="${BATSLIB_FILE_PATH_ADD-}"
-    batslib_print_kv_single 4 'path' "${file/$rem/$add}" \
-      | batslib_decorate "given user is the $owner, but it was expected not to be" \
-      | fail
-  fi
-fi
 }
 
 # Fail if the file has given permissions. This