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:
=if(Er=MyDate,"Y","")
Note that you could play around with this to say, return a "Y" if Next Test Date was within 7+/- days of MyDate
=if(AND(Er>=MyDate-7,Er<=MyDate+7),"Y","")
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.
HTH
BigC