Fix a typo and expand examples to include freeing memory
where it's not obvious

Added `g_alloca0()` which wraps `g_alloca()` and initializes
allocated memory to zeroes.

Added `g_newa0()` which wraps `g_alloca0()` in a typesafe manner.

Refreshed and tweaked by Nishal Kulkarni.

Use g_macro__has_attribute to detect it instead of
hardcoding __GNUC__ || __clang__. This adds support
for a few compiler and is consistent with the rest
of the gmacros.h file.

This constructor is useful to set options that can't change after
creation.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Closes #2472

This allows a pattern like

    g_test_message ("cannot reticulate splines: %s", error->message);
    g_test_fail ();

to be replaced by the simpler

    g_test_fail_printf ("cannot reticulate splines: %s", error->message);

with the secondary benefit of making the message available to TAP
consumers as part of the "not ok" message.

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

Forming the g_test_skip() message from printf-style arguments seems
common enough to deserve a convenience function.

g_test_incomplete() is mechanically almost equivalent to g_test_skip()
(the semantics are different but the implementation is very similar),
so give it a similar mechanism for symmetry.

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

Fix some test suite memory leaks

See merge request GNOME/glib!2195

g_source_set_name duplicates the string, and this is
showing up as one of the more prominent sources of strdups
in GTK profiles, despite all the names we use being literals.

Add a variant that avoids the overhead.