Add failing testcase for -explaintypes

This file tests that -explaintypes actually produces output on code which fails
to compile. I have altered an existing testcase, `test/files/neg/abstract`,
adding -explaintypes to the flags.

XXX: this does not work currently, as partest currently ignores the standard
output. See SI-7003.
Allow specifying extra flags for all --pos/--run testcases

The goal is to allow specifying --explaintypes for all --pos/--run files. This
is mostly the basic functionality, up for review. Once this is committed, I will
actually make use of this in another commit.
I can add such properties for all categories, if needed.