[gdb/testsuite] Reimplement gdb.gdb/complaints.exp as unittest

[gdb/testsuite] Reimplement gdb.gdb/complaints.exp as unittest

When building gdb with "-Wall -O2 -g -flto=auto", I run into:
...
(gdb) call clear_complaints()^M
No symbol "clear_complaints" in current context.^M
(gdb) FAIL: gdb.gdb/complaints.exp: clear complaints
...

The problem is that lto has optimized away the clear_complaints function
and consequently the selftest doesn't work.

Fix this by reimplementing the selftest as a unit test.

Factor out two new functions:
- void
  execute_fn_to_ui_file (struct ui_file *file, std::function<void(void)> fn);
- std::string
  execute_fn_to_string (std::function<void(void)> fn, bool term_out);
and use the latter to capture the complaints output.

Tested on x86_64-linux.