Add a test for repairing paths

tests/common.sh.in

@@ -29,6 +29,8 @@ export SHELL="@bash@"
 export version=@PACKAGE_VERSION@
 export system=@system@
 
+cacheDir=$TEST_ROOT/binary-cache
+
 readLink() {
     ls -l "$1" | sed 's/.*->\ //'
 }
@@ -55,6 +57,10 @@ clearManifests() {
     rm -f $NIX_STATE_DIR/manifests/*
 }
 
+clearCache() {
+    rm -rf "$cacheDir"
+}
+
 startDaemon() {
     # Start the daemon, wait for the socket to appear.  !!!
     # ‘nix-daemon’ should have an option to fork into the background.