[sanitizers][test] Test sanitizer_common and ubsan_minimal on Solaris

During the initial Solaris sanitizer port, I missed to enable the
`sanitizer_common` and `ubsan_minimal` testsuites.  This patch fixes this,
correcting a few unportabilities:

- `Posix/getpass.cpp` failed to link since Solaris lacks `libutil`.
  Omitting the library lets the test `PASS`, but I thought adding `%libutil`
  along the lines of `%librt` to be overkill.
- One subtest of `Posix/getpw_getgr.cpp` is disabled because Solaris
  `getpwent_r` has a different signature than expected.
- `/dev/null` is a symlink on Solaris.
- XPG7 specifies that `uname` returns a non-negative value on success.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D91606