use fixed "rand()" value for testing

when testing, instead of generating a random double, just always use
0.1. this is accomplished through the `TEST` preprocessor macro, so if
we ever want to conditionally compile based on testing vs. our main app,
we can just use:

```c
#if TEST
  // ...
#else
  // ...
#endif
```

i also changed the `test` target to always include debugging symbols. i
guess it's silly not to. though exporting `DEBUG=1` will still force
debug symbols even in the main app. note that of course we still have
the gotcha that if we compile from scratch w/o them, then `make test`,
the *.o files won't have the debug symbols, so if you're switching
contexts into debugging, you might have to run `make clean` first.

i also reverted the "Override rand() in test" commit 3b96e19d9e7680f8050561ded801fc32d964e2cb.