Rename and split elf/tst-dlopen-aout collection of tests

From the beginning, elf/tst-dlopen-aout has exercised two different
bugs: (a) failure to report errors for a dlopen of the executable
itself in some cases (bug 24900) and (b) incorrect rollback of the
TLS modid allocation in case of a dlopen failure (bug 16634).

This commit replaces the test with elf/tst-dlopen-self for (a) and
elf/tst-dlopen-tlsmodid for (b).  The latter tests use the
elf/tst-dlopen-self binaries (or iconv) with dlopen, so they are
no longer self-dlopen tests.

Tested on x86_64-linux-gnu and i686-linux-gnu, with a toolchain that
does not default to PIE.