Really sounds like you need to tell us what country you are in because I'm pretty sure that some countries would consider that this is part of your job that you do for your employer.
Let us assume you make this test gear.
Option 1: You set yourself up as an independent manufacturer (you may need to be a separate entity able to test competitor equipment or other company's products).
Now that you've sold this to your employer or to the end user, what happens when you fail a piece of equipment and it costs your (former) employer money?
Who is your client?
Who is going to make you life the most difficult?
Now you could find yourself in a three cornered legal battle as to what the whether it is your tests, your equipment or the employer's equipment that is really at fault. This may end up in litigation and cost you more than you can afford
who ever is right.
Your employer may decide in future to include an exclusion clause in their sales contract that specifically excludes any liability for or dependence on test conducted with your equipment.
At any time the employer may decide the cost is adding to the overall cost burden on a project (the customer has to pay) and decide to make their own test gear and leave you high and dry.
We could go on to construct a few different options.
In virtually any option there are just so may ways for things to go wrong that you really need to very best you can get of good will from your employer.
If they think you are taking advantage, they will mess you up good. They may let you have enough rope to hang yourself but they will mess you up.
Don't ever delude yourself that managers/employers will only ever work in the best interests of the company and that what you are doing is in the best interest of their company and you are safe. Management can turn vindictive and self destructive and get away with it. You may not survive it.
So if there is even the slightest chance that your employers will feel themselves taken advantage of, even losing profit that could have been theirs but is now yours, you can bet they may take it on the chin for the short term but mess you up in the longer term.
JMW