dev-util/rr: run tests only under FEATURE=-usersandbox

dev-util/rr: run tests only under FEATURE=-usersandbox

rr extensively uses ptrace() to set memory up for replays.
sandbox is also using ptrace() extensively.

As a result 'rr' can't use ptrace() at all:
```
[FATAL rr-5.2.0-r1/work/rr-5.2.0/src/Task.cc:2239:write_bytes_helper()
    errno: EIO]
 (task 967940 (rec:967602) at time 18)
 -> Assertion `nwritten == buf_size' failed to hold. Should have
   written 144 bytes to 0x7fff573a98a0, but only wrote -1
```

Similar to strace test suite allow running tests only under
FEATURES=-usersandbox.

This makes most tests to start passing for me.

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/632394
Package-Manager: Portage-2.3.79, Repoman-2.3.18
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>