Tests use libfmt headers without requiring or checking for its presence

[barracuda156]

CMake does not check for libfmt, but enables tests which rely on it, which in turn breaks building of tests due to non-existing header included.
See:

immer/test/oss-fuzz/flex-vector-bo-0.cpp

Line 20 in df6ef46

#include <fmt/ostream.h>