test: add an unit test for generate()

test/spasteg_test.gleam

@@ -1 +1,26 @@
+import gleam/list
+import gleam/string
+import gleeunit
+import gleeunit/should
+import key
 
+pub fn main() {
+  gleeunit.main()
+}
+
+pub fn generate_test() {
+  let key_result = key.generate()
+  let allowed_chars =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+
+  key_result |> should.not_equal("")
+
+  key_result
+  |> string.length
+  |> should.equal(16)
+
+  key_result
+  |> string.to_graphemes
+  |> list.all(fn(char) { allowed_chars |> string.contains(char) })
+  |> should.be_true
+}