When `rustdoc` tests crash, it is very difficult to investigate the failure

[nagisa]

One of the rustdoc tests is crashing and the output is:

---- src/lib.rs - span (line 28) stdout ----
Test executable failed (terminated by signal).

It is extremely difficult to actually debug this – I know rustdoc compiles a test binary and runs it, but there is no intuition whatsoever as to where to look for the executable in question!

On failures like these, I feel like we should print a path to the executable (much like cargo does).

[nagisa]

Seems like a part of the reason it is so difficult is that rustdoc places the executables into a temporary directory and this directory is removed on exit. There is a --persist-doctests option, but it is extremely undiscoverable.