I have a HttpPost methods. And I really don't know how to test it correctly.
My methods:

public static App__c ParseRequest(RestRequest req) {
    App__c app = new App__c();
    String body = req.requestBody.toString();
    app = (App__c)JSON.deserialize(body, App__c.class);
    return app;  
} 

@HttpPost
global static Id doPost() {
    RestRequest req = RestContext.request;        
    App__c app = ParseRequest(req);
    insert app;
    return app.id;
}

My test (just basis):

public static String GenerateJSON(App__c a){
    Map<String, String> fieldMap = new Map<String, String>{'Title__c' => a.Name};       
    String serialized = JSON.serialize(fieldMap);         
    return serialized;
}

@isTest
static void testPost() {
    App__c record = createTestRecord();
    RestRequest request = new RestRequest();
    request.requestUri = System.URL.getSalesforceBaseUrl().toExternalForm() + '/services/apexrest/App__c/' + record.Id;
    request.httpMethod = 'POST';
    RestContext.request = request;

    record = (App__c)JSON.deserialize(GenerateJSON(record), App__c.class);

    App__c j = Endpoint.ParseRequest(request);
    record = (App__c)JSON.deserialize(body, App__c.class);
}

static App__c createTestRecord() {
    App__c app = new App__c(
        Title__c = 'Title',
    );
    insert app;
    return app;
}

In App__c j = Endpoint.ParseRequest(request); I become Argument cannot be null. without coverage of:

app = (App__c)JSON.deserialize(body, App__c.class);
    return app;  
} 

@HttpPost
global static Id doPost() {
    RestRequest req = RestContext.request;        
    App__c app = ParseRequest(req);
    insert app;
    return app.id;
}