@available statement in a tests class has just effect for the compiler, not to skip tests contained. So you have to do it manually.

DifferentiOSVersionTests.swift

/// First option: if you have a reduced number of tests with this issue.

@available(iOS 17.0, *)
class FirstOptionTests: XCTestCase {
  func testStuffOnlyAvailableInIOS17() throws {
    guard #available(iOS 17.0, *) else {
      throw XCTSkip("Trying to run code in an unsupported iOS version")
    }
    // All the stuff just available since iOS 17.0
  }
}

/// Second option: if you have a lot of tests and you don't want to duplicate the guard

class iOS17TestCase: XCTestCase {
  override func invokeTest() {
    guard #available(iOS 17.0, *) else {
      print("Trying to run code in an unsupported iOS version")
      return
    }
    return super.invokeTest()
  }
}

@available(iOS 17.0, *)
class SecondOptionTests: iOS17TestCase {
  func testStuffOnlyAvailableInIOS17() throws {
    // All the stuff just available since iOS 17.0
  }
}