Add snapshot unit tests

Author: hrryt

DESCRIPTION

@@ -28,6 +28,10 @@ Imports:
     lifecycle,
     rlang,
     scales,
-    vctrs
+    vctrs,
+    vdiffr
 URL: https://github.com/hrryt/gglinedensity, https://hrryt.github.io/gglinedensity/
 BugReports: https://github.com/hrryt/gglinedensity/issues
+Suggests: 
+    testthat (>= 3.0.0)
+Config/testthat/edition: 3

tests/testthat.R

@@ -0,0 +1,12 @@
+# This file is part of the standard setup for testthat.
+# It is recommended that you do not modify it.
+#
+# Where should you do additional test configuration?
+# Learn more about the roles of various files in:
+# * https://r-pkgs.org/testing-design.html#sec-tests-files-overview
+# * https://testthat.r-lib.org/articles/special-files.html
+
+library(testthat)
+library(gglinedensity)
+
+test_check("gglinedensity")