[gdb/testsuite] Extend gdb.base/index-cache.exp

[gdb/testsuite] Extend gdb.base/index-cache.exp

The test-case gdb.base/index-cache.exp uses only one source file, which
contains main.

While doing "file $exec", in set_initial_language a symbol lookup of "main" is
done, causing the symtab containing main to be expanded.

Handling of main is special, and a future optimization may skip the lookup and
expansion.

Reliably exercise:
- the lookup of main, expanding the symtab containing main, by doing
  "ptype main", and
- the lookup of another symbol, expanding a symtab not containing main, by:
  - adding another source file containing function foo, and
  - doing "ptype foo".

This triggered a segfault with target board native-extended-gdbserver, filed
as PR symtab/30712, but that seems to be fixed by a previous commit in this
series.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30712