Can we use "you" to talk about people in general?
是否可以用you來代替people？

In the speaking test, it's fine to say something like "when you go on holiday, you have the chance to relax". But in a written essay, I would prefer to change this e.g. "when people go on holiday...".
在口語考試中，when you go on holiday, you have the chance to relax這樣的說法沒有問題。但在寫作考試?yán)铮腋鼉A向改成when people go on holiday。

In the speaking test, what should I do if I understand the question but I have no idea what to say in my answer?
在口語考試中，如果我聽懂了問題但不知道說什么的話該怎么辦？

You could use this formula: "Well, I don't really know much about... because..., but I suppose..." In other words, you admit that you don't really know what to say, you explain why, then you invent an answer anyway.你可以使用這個句型：Well, I don't really know much about... because..., but I suppose...。
換句話說，你承認(rèn)自己不知道說些什么，并解釋原因，然后你再虛構(gòu)一個答案。

Is it ok if my pronunciation is a mix of British and American English?
如果發(fā)音混合了英國口音和美國口音，會有問題嗎？

Yes, that's fine. Just focus on speaking clearly.
沒問題。重點(diǎn)在于表述清楚。

Can we use quotations in writing task 2 and the speaking test?
在長作文和口語考試中，是否可以引用名人名言？

Personally I don't recommend including quotations because it's difficult to use them in a natural and relevant way. It usually seems like the student has 'forced' the quotation into his/her work, and this won't impress the examiner.
我個人不推薦使用名人名言。因?yàn)槟愫茈y找到和題目相關(guān)又不生硬的名言。經(jīng)常能看到考生在文章中強(qiáng)行引用名言，這并不能給考官帶來好印象。