rtld: add some dlopen tests

[kevans]

Summary

dlopen_basic just tests that libthr.so can be dlopen()ed, which will
just serve as a sanity check that "libthr.so" is a thing that can be
dlopened in case we get a weird failure in dlopen_recursing.

dlopen_recursing tests a regression reported after the libsys split,
where some dlopen() may cause infinite recursion and a resulting crash.

[kib]

How fresh are processes where ATF executes tests? Would both dlopen_basic and dlopen_recursing execute in the same image sequentially, or ATF spawns/execs a new process to run each test?

[kevans]

In D43859#1000568, @kib wrote:

How fresh are processes where ATF executes tests? Would both dlopen_basic and dlopen_recursing execute in the same image sequentially, or ATF spawns/execs a new process to run each test?

Each individual test case within a program will always be executed in a fresh image -- ATF doesn't have any way to request multiple tests in a single execution of the test program