How to create measurement points on a PCB for MIPI lane diagnostics?
To get to MIPI signals and check channel fuctionality, the best way is to use MIPI analyzer,  like teledynelecroy.com/protocolanalyzer/mipi X34 Eclipse

There are several ways to attach signals. 

One is with a set of sophisticated solder-on probes. You will need to have a pair of simple test pads on each differential transmission line, so the probes are soldered with tiny AWG36-AWG40 wires. Round pads with diameter of 0.3-0.5mm will suffice.

The other method is to use a special spring-loaded connector. MIPI traces should be routed accordingly. Unfortunately it takes an extra space.

  

If you need to test signal eyes, there are differential probes for this, like  P7313 etc. for Tektronix 70000 series scopes. They also can be soldered to simple spot-like test points along the M-PHY diff lines.

Agilent/Keysight also offers similar tools and probes, with solderable tips like N6381A, usually with a typical with 1169A InfiniiMax amplifier:



which are usually soldered over test spots on MIPI channel.
They also have MIPI analyzers as well like U4421A model. 

But all this would cost you quite a bit.