Add more bar tests

Just making a note so I don't forget -- add more tests for 71e29b4

# https://github.com/ropensci/plotly/issues/831
df <- data.frame(
  V1 = gl(6,20), V2 = gl(40,3)
)
p <- ggplot(df, aes(x = V1, fill = V2)) + geom_bar(color="white")
ggplotly(p)


# https://github.com/ropensci/plotly/issues/560
d <- data.frame(
  x = LETTERS[1:10], 
  y = -5:4, 
  g = rep(c('A','B'), each = 5)
)

p <- ggplot(data = d, aes(x = x, y = y, fill = g)) +
  geom_bar(stat='identity')
ggplotly(p)

# https://github.com/ropensci/plotly/issues/901
data_before <- data.frame(
  year = c(2014,2014,2014,2015,2015,2015,2016,2016,2016),
  pet = c("dog","cat","bird","dog","cat","bird","dog","cat","bird"),
  wt = c(56, 25, 26, 10, 19, 41, 16, 17, 13)
)
data_after <- data.frame(
  year = c(2014,2014,2014,2015,2015,2015,2016,2016,2016),
  pet = c("dog","cat","bird","dog","cat","bird","dog","cat","bird"),
  wt = c(49, 18, 19,  3, 12, 34,  9, 10,  6)
)
p <- ggplot() +
  geom_bar(data = data_before, aes(year, wt, fill = pet), 
           stat = "identity", position = "dodge", width = 0.9, alpha = 0.5) +
  geom_bar(data = data_after, aes(year, wt, fill = pet), 
           stat = "identity", position = "dodge", width = 0.9) +
  labs(x = "Year", y = "Weight") +
ggplotly(p)

# https://github.com/ropensci/plotly/issues/874
dd <- data.frame(x = rnorm(100,5,2))
dd2 <- data.frame(x = rnorm(100,5,2))
p3 <- ggplot() + 
  geom_histogram(data = dd,aes(x = x,y = ..count..)) + 
  geom_histogram(data = dd2,aes(x = x,y = -..count..),color = 'green')
ggplotly(p3)



# https://github.com/ropensci/plotly/issues/968

p <- ggplot(data=dat, aes(x=time, y=total_bill)) +
  geom_bar(stat="identity") + coord_flip()
ggplotly(p)

No test file or entry point is named. Start by locating the existing ggplotly bar and histogram tests, then review the reproduction examples linked to issues 831, 560, 901, 874, and 968; done means each case has regression coverage with its expected plot behavior recorded.