Improve decompilation tests

[pfez]

This PR does 3 things:

• enables the -O2 optimization flags on binaries intended for decompilation tests. We want those to be optimized because we want the decompiler to give good results on high optimization levels.
• avoid calling malloc to work around some current limitations/bugs of revng, without affecting the quality of the decompilation test
• adds some accessors to the linked list test, to allow for more interesting results in decompilation.

[aleclearmind]

Looks good. Will go in soon!

[aleclearmind]

Merged in 4d37a31. Thanks!