test/-ext-/string/test_modify_expand.rb: increase limit for {je,tc}malloc

In this test, jemalloc and tcmalloc are both more hesitant to
release memory to the kernel than the stock glibc allocator.

Tested on jemalloc 3.6.0 (self-built) and libtcmalloc-minimal 2.0-2
(Debian package) on x86_64-linux

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47620 b2dd03c8-39d4-4d8f-98ff-823fe69b080e