add visual tests

Author: teunbrand

tests/testthat/_snaps/geom-curve/flipped-geom-curve.svg

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+test_that("geom_curve flipping works", {
+
+  df <- data.frame(x = c(1, 2), xend = c(2, 3), y = 1, yend = c(2, 1.5))
+
+  p <- ggplot(df, aes(x, y, xend = xend, yend = yend)) +
+    geom_curve(arrow = arrow())
+
+  expect_doppelganger("standard geom_curve", p)
+  expect_doppelganger("flipped geom_curve", p + scale_y_reverse())
+
+})