they're very fragile by nature, but let's at least move them
into one file with msan/embedded/ps/32-bit all disabled to
have the memory usage more predictable.
And remove these restrictions from other test files.
(Variant 2: use stack for buffers)
my_malloc_size_cb_func() has a call to thd->alloc() to produce an error message.
thd->alloc() calls alloc_root(), so one can end up with this stack trace:
alloc_root()
THD::alloc()
my_malloc_size_cb_func()
my_malloc()
alloc_root()
where alloc_root() calls itself. This is a problem, as alloc_root() is not
reenterable.
Fixed this by switching my_malloc_size_cb_func() to use space on the stack
instead.
