The /testfor command is essentially useless in accomplishing that, since all it provides you is whether or not there was a match.
You will instead need to use the /scoreboard command to assign a label to targets that match NBT data, and you'd then be able to target them based on the label in other commands. You can either use a score or a "tag".
##Score
You will first need to create an objective to hold the scores in. You'd run this a single time yourself in the chat:
/scoreboard objectives add assassinKit dummy
You may then use the following on a clock, which first sets player's "assassinKit" score to 0 by default, in case they take the armor off, and then sets it to 1 afterwards if they are still wearing the particular armor set:
/scoreboard players set @a[score_assassinKit_min=1] assassinKit 0
/scoreboard players set @a assassinKit 1 {Inventory:[{id:minecraft:leather_boots,Damage:0s,Count:1b,Slot:100b},{id:minecraft:leather_leggings,Damage:0s,Count:1b,Slot:101b},{id:minecraft:leather_chestplate,Damage:0s,Count:1b,Slot:102b},{id:minecraft:leather_helmet,Damage:0s,Count:1b,Slot:103b}]}
And finally, target them based on their score:
/say Players wearing the assassin kit: @a[score_assassinKit_min=1]
/effect @a[score_assassinKit_min=1] minecraft:speed
##Tag
A "tag" is a very simple string label that requires no objective, and is stored on the target among a list of other "tags". Same as before and on a clock, you'd remove it by default first, and then re-apply it:
/scoreboard players tag @a[tag=assassinKit] remove assassinKit
/scoreboard players tag @a add assassinKit {Inventory:[{id:minecraft:leather_boots,Damage:0s,Count:1b,Slot:100b},{id:minecraft:leather_leggings,Damage:0s,Count:1b,Slot:101b},{id:minecraft:leather_chestplate,Damage:0s,Count:1b,Slot:102b},{id:minecraft:leather_helmet,Damage:0s,Count:1b,Slot:103b}]}
And finally, target them based on their tag:
/say Players wearing the assassin kit: @a[tag=assassinKit]
/effect @a[tag=assassinKit] minecraft:speed
The disadvantage of tags compared to scores is the increased difficulty in targeting something based on multiple tags. All selector parameter keys must be unique, so @e[tag=1,tag=2] does not work (selecting only players with the last-specified unique key). You would instead have to add another tag (or score) based on the target's Tags NBT list:
/scoreboard players tag @a[tag=hasMultipleTags] remove hasMultipleTags
/scoreboard players tag @a[tag=tag1] add hasMultipleTags {Tags:["tag2","tag3"]}
/say Players with tags "tag1", "tag2", and "tag3": @a[tag=hasMultipleTags]
