Do not ship/install "test_all"

[paravoid]

The "test_all" binary is being shipped when one runs "make install". The issue is that it's configured in automake as a binary, rather than as a check. The fix is pretty trivial: in src/Makefile.am, one can do:

-bin_PROGRAMS = test_all
+check_PROGRAMS = test_all

(I do not consider the above contribution as copyrightable. No, I'm not going to sign a CLA or any other kind of agreement or contract with Twitter under any terms, hence no PR.)