Use /tmp directory as a default one during testing

Since `/tmp` is used as a working directory for BATS tests, there is no
chance that any unexpected modifications will happen within mounted
project directory.

Also, the `/tmp` does not have an initialized git repository. This is
good as if we need a real repository, we have to use addon for it.

tests/addons-common.bash

@@ -4,6 +4,8 @@ set -e
 THIS="$( cd -P "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 BIN_DIR="$THIS/../libexec"
 export PATH=${BIN_DIR}:$PATH
+# By default, all execution should work here.
+cd /tmp
 
 check(){
     run "$@"