How can I get XCTest to wait for async calls in setUp before tests are run?

Rather than using semaphores or blocking loops, you can use the same waitForExpectationsWithTimeout:handler: function you use in your async test cases.

// Swift
override func setUp() {
    super.setUp()

    let exp = expectation(description: "\(#function)\(#line)")

    // Issue an async request
    let data = getData()
    db.overwriteDatabase(data) {
        // do some stuff
        exp.fulfill()
    }

    // Wait for the async request to complete
    waitForExpectations(timeout: 40, handler: nil)
}

// Objective-C
- (void)setUp {
    [super setUp];

    NSString *description = [NSString stringWithFormat:@"%s%d", __FUNCTION__, __LINE__];
    XCTestExpectation *exp = [self expectationWithDescription:description];

    // Issue an async request
    NSData *data = [self getData];
    [db overwriteDatabaseData: data block: ^(){
        [exp fulfill];
    }];        

    // Wait for the async request to complete
    [self waitForExpectationsWithTimeout:40 handler: nil];
}