tests: slabinfo should not be too strict about slab names

tests: slabinfo should not be too strict about slab names

Before this commit, the test checking `vmstat -m` (slabinfo) output uses
a fairly strict regular expression that only allows alphanumeric
characters and a few exceptions such as "_", "-", "(" and ")".

However, recent kernels use a wider range of characters, such as ">".
For instance, see this Linux commit which creates a "page->ptl" slab:
  https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/mm/memory.c?id=b35f1819acd9243a3ff7ad25b1fa8bd6bfe80fb2#n4283

Other patches for reporting slab usage per memcg include the names of
the cgroup in the slabinfo output, which can include additional
characters and use dots for abbreviation.

The check should not be so string, instead it could simply look for a
chain of non-whitespace characters and that should be enough.

Tested that `make check` is still working, including in some of the
environments where features that enable the additional slabinfo names.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Craig Small <csmall@enc.com.au>