Tighten up the exportReferencesGraph tests

Add an `$` at the end of the `grep` regex. Without it, `checkRef foo`
would always imply `checkRef foo.drv`. We want to tell these situations
apart to more precisely test what is going on.

tests/export-graph.sh

@@ -4,7 +4,7 @@ clearStore
 clearProfiles
 
 checkRef() {
-    nix-store -q --references $TEST_ROOT/result | grep -q "$1" || fail "missing reference $1"
+    nix-store -q --references $TEST_ROOT/result | grep -q "$1"'$' || fail "missing reference $1"
 }
 
 # Test the export of the runtime dependency graph.