Fix: .gitignore: use full paths for test binaries

Using only the binary name in the .gitignore paths yields confusion
for git and search tools relying on this file's content (e.g. ag
silversearcher), because the binary file name may also be part of a
path, e.g. the filename "gen-ust-events" is also part of the path
leading to the .c file implementing this test:
"./tests/utils/testapp/gen-ust-events/gen-ust-events.c", which means
this .c file is ignored, even though it should not be.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I309b268fed8e66b693c1e0670044b40bf90593c4