Tests: Add a unit test for vfs.dlopen error handling

Doesn't cover the main functionality; which is just too involved for a mere unit test.

Tests/BDD/vfs-library.spec.lua

@@ -133,4 +133,11 @@ describe("vfs", function()
 		end)
 	end)
 
+	describe("dlopen", function()
+		it("should throw if an invalid library name was passed", function()
+			assertThrows(function()
+				vfs.dlopen({}, nil)
+			end, "Expected argument libraryName to be a string value, but received a nil value instead")
+		end)
+	end)
 end)