Improve plotly+shinytest experience

[cpsievert]

This PR addresses the main problem in rstudio/shinytest#174, but it's worth noting there are a couple remaining issues to be aware of:

• Since shinytest currently runs on phantomjs, it won't work well with plotly's WebGL chart types. a9714dd adds a warning if plotly detects webgl in shinytest mode, but until recordTest() should capture and print errors from Shiny process rstudio/shinytest#146 is addressed, there is no real way for users to see that warning
• How should input values that are triggered through direct manipluation events be handled? I suppose a more long-term solution would be to leverage whatever comes out of Idea: add a custom input binding that allows users to set arbitrary values rstudio/shinytest#193, but a shorter-term fix might be to use shiny::snapshotPreprocessInput()?

Testing notes

Install devtools::install_github("ropensci/plotly#1379"), then use shinytest to record and test the following app -- https://github.com/ropensci/plotly/tree/master/inst/examples/shiny/Diamonds

Make sure you don't get false positive test failures when running the tests.

Here is some code to help with those tasks:

git clone https://github.com/ropensci/plotly.git
cd plotly
R
shinytest::recordTest("inst/examples/shiny/Movies/")
shinytest::testApp("inst/examples/shiny/Movies/")

[wch]

This isn't important, but I find it slightly clearer to use setdiff(), as in:

 json$x[setdiff(names(json$x), c("visdat", "cur_data", "attrs"))]