[gdb/testsuite] Improve failure mode in gdb.dwarf2/enqueued-cu-base-addr.exp

[gdb/testsuite] Improve failure mode in gdb.dwarf2/enqueued-cu-base-addr.exp

I ran test-case gdb.dwarf2/enqueued-cu-base-addr.exp with target board
cc-with-debug-names, and ran into:
...
FAIL: gdb.dwarf2/enqueued-cu-base-addr.exp: ptype foo (GDB internal error)
FAIL: gdb.dwarf2/enqueued-cu-base-addr.exp: CU addr found
...

The first FAIL is a known issue, PR symtab/29572.

The following FAIL is a consequence of the first FAIL, so require for the
second test that the first test passes.

Tested on x86_64-linux, with target boards unix and cc-with-debug-names.