Listening
"As you listen, write your answers on the question paper. At the end of the test, you will have 10 minutes to transfer your answers to the answer sheet in pencil. You may write your answers in lower case or capital letters."
So, you can write your answers entirely with capital letters or all in lower case - except if the answer requires a capital letter, for example, a name, or a city name.
For example: LONDON or London but not london
Reading
The website does not specify for the reading test but it is generally assumed to be the same - use all capitals or all lower case, expect when a capital letter is needed. Many of the answers in the reading test come directly from the reading paper so copy it directly from there and you can't go wrong.
Writing
"Write your answers in pen or pencil. You may write entirely in capital letters if you wish."
The website is clear on this: you can write entirely in capital letters if you want to. I personally would find this hard to do but some people do prefer to write in capital letters and I suppose it solves the issue of making mistakes where you are not sure whether to use a capital letter or not - because this would be considered in the 'Grammatical Range and Accuracy' section of the marking.
If you are going to use lower case and capital letters only where necessary, here is a reminder of when to use capitals:
To start a sentence: The housing market crashed, causing a financial crisis.
For proper nouns:
Names of people and titles - Joe, Mr Dougherty, Queen Elizabeth II
Names of villages, towns, cities, counties, countries, continents - Addingham, Urmston, Manchester, Lancashire, England, Europe
Languages and nationalities - English and the English
Monuments and famous places - The Eiffel Tower, the London Eye
Days, months, holidays: Monday, April, Christmas
Religions and religious people: Islam, Muslim
Rivers, lake, mountains, deserts: River Irwell, Windermere Lake, Mount Everest, the Dolomites
Trademarks, companies, organisations: Nike, Peugeot, the United Nations
Ship/ train/ aircraft names: The Titanic, the Orient Express, the Enterprise
Names of books, poems, songs, albums, plays and films: The Bible, Jabberwocky, Imagine, Stars, Romeo and Juliet, The Truman Show
Acronyms - UK, NATO
The start of speech - He said, "Why don't we go out?"
Personal pronoun 'I' - I believe education is the most important part of a person's life.
Titles - The importance of capital letters
Vocabulary
clarify (v) = make it clear, explain
specify (v) = be specific about, refer to
reminder (n) = something to make you remember
entirely (adv) = completely, totally
cause (v) = lead to, make
acronym (n) = a shortened version, using just the first letter (capitalised) of each word
Good luck and please ask if you have any more doubts regarding the capital letters. I would say, however, if in doubt, write ALL in capitals! But be consistent, do not write one word in capitals and not the next.