[gdb/testsuite] Simplify gdb.base/unwind-on-each-insn.exp

In test-case gdb.base/unwind-on-each-insn.exp, we try to determine the last
disassembled insn in function foo.

This in it self is fragile, as demonstrated by commit 91836f41 ("Powerpc
fix for gdb.base/unwind-on-each-insn.exp").

The use of the last disassembled insn in the test-case is to stop stepping in
foo once reaching it.

However, the intent is to stop stepping just before returning to main.

There is no guarantee that the last disassembled insn:
- is actually executed
- is executed just before returning to main
- is executed only once.

Fix this by simplying the test-case to continue stepping till stepping out of
foo.

Tested on x86_64-linux.