Restore previous XCTVaporTests behavior if necc.

Sources/XCTVapor/XCTVaporTests.swift

@@ -1,18 +1,45 @@
-public var app: (() throws -> Application) = {
-    fatalError("implement static app generator")
-}
+/**
+ We recommend configuring this in your test’s `class func setUp()`
+ */
+public var app: (() throws -> Application)!
 
 open class XCTVaporTests: XCTestCase {
     open var app: Application!
-    
+
     open override func setUpWithError() throws {
+
+        // The XCTest runner calls this function before setUp()
+
+        try super.setUpWithError()
+
+        // this optional check due to prior usage by users
+        // see: https://github.com/vapor/vapor/pull/2585#issuecomment-802144636
+        if let _app = XCTVapor.app {
+            self.app = try _app()
+        }
+    }
+
+    open override func setUp() {
+
+        // The XCTest runner calls this after setupWithError()
+
         super.setUp()
-        self.app = try XCTVapor.app()
+
+        if self.app == nil, let _app = XCTVapor.app {
+            // this was the behavior of this class pre 4.41.5
+            // keeping for compatability however it will crash obv if
+            // the function throws. Provided the user assigns to
+            // XCTVapor.app in the class setUp everything will work
+            // as we intend.
+            self.app = try! _app()
+        } else {
+            fatalError("implement static app generator")
+        }
     }
-    
+
     open override func tearDown() {
         super.tearDown()
         self.app?.shutdown()
-        app = nil
+        self.app = nil
     }
 }