Previously it would silently fail to create the /gnu/store symlink when
the host has a read-only /gnu as is the case in these tests.
* gnu/packages/aux-files/run-in-namespace.c (exec_with_loader): Unlink
the ancestor of ORIGINAL_STORE under NEW_ROOT. Check the return value
of 'symlink' when creating NEW_STORE.
* tests/guix-pack-relocatable.sh: Check the contents of the store as
seen by the wrapped executable, with all three engines, and with both
"/gnu" and "/gnu/store" erased.
