Fix example test environment file in testing doc (#1056)
Fix example test environment file in testing doc (#1056)

Copying the example verbatim without ensuring "toWebpackConfig()" is present results in strange errors trying to run tests.
gauravtiwari/webpacker,rossta/webpacker,rails/webpacker,rossta/webpacker,gauravtiwari/webpacker,rossta/webpacker,rails/webpacker,rossta/webpacker,gauravtiwari/webpacker