Improve interaction between ctrl-c and Test()

[fingolfin]

Right now, pressing ctrl-c while Test(...) is running does not open a break prompt, nor does it abort Test(...). Instead, the user interrupt is treated like an error in the code being tested. This makes it difficult to abort certain long running tests; sometimes, if you are lucky, you can press ctrl-c quickly repeatedly to at least exit GAP, but even that can fail.

Perhaps we could teach Test() and friends to treat user interrupts differently -- i.e. if a user interrupt happens, either abort Test(), or (for bonus points) actually enter a break prompt (that might be difficult to pull of, though; it's not that important anyway).

To implement this, I imagine we'd modify (at least) the following places:
1 . READ_TEST_OR_LOOP, to teach it to abort upon user interrupt;
2. FuncREAD_STREAM_LOOP, to signal whether the operation completed normal, or was interrupted by the user (and possibly more), e.g. via its return value
3. RunTests in lib/test.gi then would track the return value of READ_STREAM_LOOP, and if it indicates a user interrupt, would take appropriate steps, e.g. abort itself, and possibly indicate that via a return value, too. Or perhaps better, it could just issue an Error("user interrupt detected") of its own, with the break loop re-enabled; then the user can choose whether to abort the test (via quit;), or to continue to the next test file (by doing a return;; then I imagine right after the Error, RunTests would return itself, so that processing the current file ends).