Add a test to confirm the prefix doesn't work fine with `/bin/ruby`

test/prefix.bats

@@ -24,6 +24,15 @@ load test_helper
   assert_success "$RBENV_TEST_DIR"
 }
 
+@test "prefix for system in /" {
+  mkdir -p "${RBENV_TEST_DIR}/bin"
+  touch "${RBENV_TEST_DIR}/bin/rbenv-which"
+  echo "echo /bin/ruby" >"${RBENV_TEST_DIR}/bin/rbenv-which"
+  chmod +x "${RBENV_TEST_DIR}/bin/rbenv-which"
+  RBENV_VERSION="system" run rbenv-prefix
+  assert_success "/"
+}
+
 @test "prefix for invalid system" {
   PATH="$(path_without ruby)" run rbenv-prefix system
   assert_failure "rbenv: system version not found in PATH"

test/test_helper.bash

@@ -20,10 +20,10 @@ if [ -z "$RBENV_TEST_DIR" ]; then
   export RBENV_HOOK_PATH="${RBENV_ROOT}/rbenv.d"
 
   PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
-  PATH="${RBENV_TEST_DIR}/bin:$PATH"
   PATH="${BATS_TEST_DIRNAME}/../libexec:$PATH"
   PATH="${BATS_TEST_DIRNAME}/libexec:$PATH"
   PATH="${RBENV_ROOT}/shims:$PATH"
+  PATH="${RBENV_TEST_DIR}/bin:$PATH"
   export PATH
 
   for xdg_var in `env 2>/dev/null | grep ^XDG_ | cut -d= -f1`; do unset "$xdg_var"; done