Improve test debugging

[mahkoh]

Currently, the best way to debug a test is to comment out all other tests and then to set follow-fork-mode child in gdb. This can be done better.

[mahkoh]

It's now possible to run make gdb which will set gdb up to follow the child. You can also pass arguments to the executable and it will then only run those tests that start with one of the arguments. For example

r vec

Will run all tests in the vec module.

r vec::drain

Will only run the vec::drain test.

[mahkoh]

But note that you can only debug one test at a time in gdb because it always follow the first child process.