test: Explicitly enable assertion macros

To allow dbus-glib to be built with G_DISABLE_ASSERT defined but tests
enabled (--disable-asserts --enable-tests), we need to explicitly
undefine it when building the tests, otherwise g_test_init() turns
into an abort since GLib 2.58.

We no longer actually use g_assert() for test-only code, so we can
remove the warning about tests' failing results not being reported.

test-profile.c still leaves assertions disabled: it does not use
g_test_init(), and it is intended to be used for profiling, where the
overhead of assertions is potentially significant.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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