A test laboratory establishes the properties of something. How many bacteria are there in the meat? How much harmful radiation does a device emit? How many heavy metals are in the water? This is often established using measuring equipment. This equipment therefore has to be calibrated by a calibration laboratory. The test results are supplied to the client who then determines the meaning of the results. Is this brick hard enough for road use? Is this drinking water suitable for consumption? And so on. In principle, everything that can be measured is tested by a laboratory. The field of work of a test laboratory is therefore broad.
The RvA accredits test laboratories on the basis of the ISO/IEC 17025 standard. Accredited laboratories are obliged to participate in proficiency testing: testing whereby the test results from various laboratories are compared against each other.
You can use the search engine or consult the list of accredited bodies to find details of all test laboratories accredited by the RvA. Policy document BR010 describes the various fields of work relating to testing.