Drop a questionable test from the refstring suite

Testing that an interned string is released once its last reference is
dropped is not possible without a hook into the interning machinery;
just checking that the returned pointer for the same string is going to
be different after the last release() is not guaranteed to work, as the
systema allocator is perfectly within its rights to recycle pointers, as
long as they are unique while valid.

Closes #1467
8 jobs for issue-1467 in 10 minutes and 23 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #76949
cross-android_api21_arm64

00:07:28

passed #76950
cross-android_api28_arm64

00:07:34

passed #76951
cross-mingw64

00:08:14

passed #76948
fedora-x86_64

00:05:57

manual #76954
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #76952
win32
msys2-mingw32

00:06:36

passed #76953
win32
vs2017-x64

00:06:02

 
  Coverage
passed #76955
coverage

00:00:59

73.3%