Tests: Add UnitTests folder

Tests/UnitTests/DocumentTests/LetterheadTest.cs

@@ -0,0 +1,24 @@
+using Elwig.Documents;
+using Elwig.Helpers;
+
+namespace Tests.UnitTests.DocumentTests {
+    [TestFixture]
+    public class LetterheadTest {
+
+        [Test]
+        public async Task Test_01_SimpleLetterhead() {
+            using var ctx = new AppDbContext();
+            var m = await ctx.Members.FindAsync(104);
+            using var doc = new Letterhead(m!);
+            var text = await Utils.GeneratePdfText(doc);
+            Assert.Multiple(() => {
+                Assert.That(text, Contains.Substring("WG Test | Genossenschaftsstraße 1 | 2120 Wolkersdorf"));
+                Assert.That(text, Contains.Substring("""
+                    WINZER Waltraud
+                    Wiener Straße 15
+                    2120 Wolkersdorf im Weinviertel
+                    """));
+            });
+        }
+    }
+}