[tests] Define unbound variables in test scripts

tests/test_cli.sh

@@ -4,6 +4,7 @@
 set -eu
 
 export BEAKERLIB_DIR=$(mktemp -d /tmp/composer-test.XXXXXX)
+CLI="${CLI:-}"
 
 function setup_tests {
     local share_dir=$1

tests/testenv.sh

@@ -1,5 +1,9 @@
 #!/bin/sh
 
+top_srcdir="${top_srcdir:-}"
+top_buildir="${top_builddir:-}"
+PYTHONPATH="${PYTHONPATH:-}"
+
 if [ -z "$top_srcdir" ]; then
     echo "*** top_srcdir must be set"
     exit 1