Dashr test support

[byronz]

This fixes #826

the app as string sample

app = '''
library(dash)
library(dashHtmlComponents)
app <- Dash$new()

app$layout(htmlDiv(list(htmlDiv(id='test',children='hello test'))))
app$run_server()
'''

def test_r001_r_with_string(dashr):
    dashr.start_server(app)

the filepath version

def test_r002_r_with_path(dashr):
    dashr.start_server('/Users/byron/code/demo.R')

[byronz]

@rpkyle @Marc-Andre-Rivet I need a review on this

[rpkyle]

💃 LGTM!

[alexcjohnson]

Seems like this will work, but it doesn't remove the file afterward, does it? Would there have been a way to do this with tmpdir? Not sure how to go about using one fixture from within the definition of another, but it feels like all the edge cases and cleanup have already been sorted out there.