[gdb/testsuite] Fix error handling in gdb_file_cmd

[gdb/testsuite] Fix error handling in gdb_file_cmd

Consider a gdb_load patch to call the gdb_file_cmd twice:
...
 proc gdb_load { arg } {
     if { $arg != "" } {
+       set res [gdb_file_cmd $arg]
+       if { $res != 0 } {
+           return $res
+       }
        return [gdb_file_cmd $arg]
     }
     return 0
 }
...

When running test-case gdb.base/index-cache.exp, we run into:
...
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache, other program \
  already loaded (timeout).
FAIL: gdb.base/index-cache.exp: test_cache_enabled_miss: check index-cache \
  stats (GDB internal error)
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache, other program \
  already loaded (timeout).
...

The first timeout in more detail:
...
(gdb) file outputs/gdb.base/index-cache/index-cache^M
Load new symbol table from "index-cache"? (y or n) y^M
Reading symbols from index-cache...^M
src/gdb/dwarf2/read.c:2540: internal-error: \
  void create_cus_from_index(dwarf2_per_bfd*, const gdb_byte*, offset_type, \
                             const gdb_byte*, offset_type): \
  Assertion `per_bfd->all_comp_units.empty ()' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
Quit this debugging session? (y or n) ERROR: Couldn't load index-cache, \
  other program already loaded (timeout).
...

Proc gdb_file_cmd has a gdb_expect handling the result of the file command,
and if the result is a "Load new symbol table from index-cache? (y or n) "
prompt, it sends a "y" and enters in a nested gdb_expect to handle the
result.

The first gdb_expect contains code to handle "A problem internal to GDB has
been detected", but the second one doesn't, which causes the timeout.

Fix this by removing the nested gdb_expect, and using exp_continue instead,
such that we have instead:
...
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache -- with new \
  symbol table into gdb (GDB internal error).
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache -- with new \
  symbol table into gdb (GDB internal error).
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-04  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_file_cmd): Replace incomplete gdb_expect by
	exp_continue.