Tester Perfumes are manufactured by the original designer of the fragrances for promotional purposes. TESTERS are actually intended for customer testing of the product in retail stores. Contents of all TESTERS are exactly the same as the contents of the normal retail version and come in exactly the same bottle as the retail version. TESTERS come in plain brown or white packaging (no fancy packaging) and the spray bottles may not come with a cap or lid. There is also a label "TESTER/DEMONSTRATION", "TESTER NOT FOR SALE" or something to that effect in both the box and bottle.
Are TESTERS fake? Definitely not, they are the real thing! The only drawback for TESTERS is that they are not appropriate to be given as gifts because they come in plain brown or white packaging and they have a "TESTER" label. For all other intensive purposes (especially for personal use) they are great, since they are 100% authentic (minus the fancy packaging) and yet a lot cheaper.
However, please note that there are now unscrupulous sellers who are selling what they claim as US Authentic Testers but which are in fact FAKE perfumes labeled as TESTERS. In case you are buying from sellers who you do not trust, please be wary, and buy only perfumes you have used before and are familiar with so you wouldn't be fooled.