tests/kolainst: Avoid recursive symlinks

`kola` now follows symlinks when archiving an external test's `data/`
dir. So the recursive `data` symlink we have here breaks it.

Let's just move the shared files in its own directory and update the
symlinks.

tests/kolainst/destructive/data

@@ -1 +1 @@
-..
+../data-shared

tests/kolainst/nondestructive/data

@@ -1 +1 @@
-..
+../data-shared

tests/kolainst/nondestructive/libtest-core.sh

@@ -1 +1 @@
-../libtest-core.sh
+../data-shared/libtest-core.sh

tests/libtest-core.sh

@@ -1 +1 @@
-kolainst/libtest-core.sh
+kolainst/data-shared/libtest-core.sh