tests: Print the failing line numbers when a test fails.
Currently, if a probe fails, it will print the line number of the [test] block the probe is in, not the line number of the probe itself. This makes it somewhat difficult to debug.
This commit makes it print the line number that a test fails at.
This code was actually written by @hverbeet for my first attempt at fixing this, !499 (closed).
CC @zfigura