What does "testing ground" mean?

Like this sentence, I'm curious about the way to use the expression "of + [someone]".

There are a couple of expressions that use "of + [someone]." One of them is <adjective + of + someone>. Another is <on behalf + of + someone> which means that you represent someone - as if in place of them. <Adjective + of + someone>, as used in this video, describes someone in relation to something they did. Ex: That was terrible of me to say! I'm so sorry. Ex: That's kind of you to help the school paint the art room! Ex: It's good of Rachel to be so considerate of Terry. Ex: As part of their company, we accept this award on behalf of the band. Ex: On behalf of my brother, I would like to say thank you for being here.

I heard that mistletoe symbolizes a good superstition for Christmas. In contrast, is there any superstition that symbols a bad jinx for Christmas?

Good question! Although many people don't really believe in these superstitions today, people in the past believed that wearing new shoes on Christmas Day would bring you bad luck, that ending a year in debt (owing someone money or possessions) means a whole new year of debt, and that leaving your Christmas decorations up past January 5th was also bad luck. Christmas and New Year's are related holidays, so many superstitions were related to them as well.

Why only "sunshine" is placed after "amount of"? In what kind of situation should I put an "amount of"?

"Amount(s) of" is used to describe a unit that can be measured, and sunshine can be measured in terms of hours. For example, two hours of sunshine in a day. Technically, rainfall can also be measured in units, so "amounts of rainfall" can be used in this sentence too. In reference to "temperatures," we cannot use "amounts of" as the unit is already implied (degrees), so "amounts" is not necessary here. Ex: We will have an average amount of rainfall this year. Ex: California is experiencing a very above average number of forest fires this year.

I think I've heard the term "French Cajun" before. Does it related to this food "cajun?"

Yes, it does relate to "Cajun food." "Cajun food" comes from "Cajun" people's culture, who are descendants of the french! So, while "Cajun" refers to the people who are part of the group and origin, it's also an adjective used to describe their culture. Ex: I really enjoy listening to Cajun music. Ex: Elie is Cajun and knows a bit of French.

Which term is more commonly used? A "merry-go-round?" Or a "carousel?"

That depends where you are! "Merry-go-round" is considered a British term, and "carousel" is considered American English. So you could use either one, but other people's understanding could differ depending on location. Some people consider a "merry-go-round" something without a motor or horses, and where you just spin people. Ex: Let's go on the carousel! = Let's go on the merry-go-round! Ex: I love seeing all the horses on the carousel. Ex: Can you spin me on the merry-go-round?