There will be several ways you can do this. What I would do is:
Equipment name in column A
Testing frequency (in days) in column B
Last Date Tested in column C
Next Date for Testing in column D includes following formula: = Br+Cr (where r= row No.)
Due for Testing? in column E includes following formula:
Note that you could play around with this to say, return a "Y" if Next Test Date was within 7+/- days of MyDate
Then just apply Autofilters to your column headings, and select "Y" from the drop-down list on column E will give you all items due for testing.
Set your print area to include ALL items of equipment (no filter), and you can then print the filtered list whenever applied.