Use create_hook helper

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

test/version-name.bats

@@ -25,13 +25,9 @@ setup() {
 @test "RBENV_VERSION can be overridden by hook" {
   create_version "1.8.7"
   create_version "1.9.3"
+  create_hook version-name test.bash <<<"RBENV_VERSION=1.9.3"
 
-  mkdir -p "${RBENV_ROOT}/rbenv.d/version-name"
-  cat > "${RBENV_ROOT}/rbenv.d/version-name/test.bash" <<HOOK
-RBENV_VERSION=1.9.3
-HOOK
-
-  RBENV_VERSION=1.8.7 RBENV_HOOK_PATH="${RBENV_ROOT}/rbenv.d" run rbenv-version-name
+  RBENV_VERSION=1.8.7 run rbenv-version-name
   assert_success "1.9.3"
 }