Add failing unit test in the single file package

to check if unit tests from the main source file run at all. Because by default the main source file is excluded from tests.

test/issue_2051_running_unittests_from_dub_single_file_packages_fails.d

@@ -67,5 +67,33 @@ unittest
 	else
 		writeln("\nOk. Unittest passed.");
 
-	return rc1;
+	// Check if dub `test` command runs unittests for single file package
+	{
+		filename = tempDir.buildPath("issue2051_fail.d");
+		auto f = File(filename, "w");
+		f.write(
+`#!/usr/bin/env dub
+/+ dub.sdl:
+	name "issue2051"
++/
+
+version(unittest) {}
+else void main()
+{
+}
+
+unittest
+{
+	assert(0);
+}
+`		);
+	}
+
+	const rc2 = text(dub, " test --single ", filename).executeCommand;
+	if (rc2)
+		writeln("\nOk. Unittests failed.");
+	else
+		writeln("\nError. Unittest passed.");
+
+	return rc1 | !rc2;
 }