test: ensure NOT_CRAN="true" on GitHub Actions (#349)

Author: eitsupi

tests/testthat/setup.R

@@ -0,0 +1,7 @@
+# Ensure running snapshot tests on GitHub Actions CI
+if (identical(Sys.getenv("GITHUB_ACTIONS"), "true")) {
+  withr::local_envvar(
+    list(NOT_CRAN = "true"),
+    .local_envir = teardown_env()
+  )
+}