functional-tests: skip tests if the kernel restricts unprivileged user namespaces

Update tests/functional/common/functions.sh

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: bryango <bryango@users.noreply.github.com>
(cherry picked from commit da7f7ba810)

tests/functional/local-overlay-store/bad-uris.sh

@@ -19,7 +19,7 @@ TODO_NixOS
 
 for i in "${storesBad[@]}"; do
     echo $i
-    unshare --mount --map-root-user bash <<EOF
+    execUnshare <<EOF
         source common.sh
         setupStoreDirs
         mountOverlayfs

tests/functional/local-overlay-store/common.sh

@@ -93,10 +93,6 @@ initLowerStore () {
   pathInLowerStore=$(nix-store --store "$storeA" --realise $drvPath)
 }
 
-execUnshare () {
-  exec unshare --mount --map-root-user "$SHELL" "$@"
-}
-
 addTextToStore() {
   storeDir=$1; shift
   filename=$1; shift