[gdb/testsuite] Fix captured_command_loop breakpoint in selftests

When building gcc with CFLAGS/CXXFLAGS="-O2 -g", and running the regression
tests, I run into the following FAILs:
...
FAIL: gdb.gdb/complaints.exp: breakpoint in captured_command_loop
FAIL: gdb.gdb/python-interrupts.exp: breakpoint in captured_command_loop
FAIL: gdb.gdb/python-selftest.exp: breakpoint in captured_command_loop
...

The problem is that when setting the breakpoint at captured_command_loop:
...
(gdb) break captured_command_loop^M
Breakpoint 1 at 0x4230ed: captured_command_loop. (2 locations)^M
(gdb) FAIL: gdb.gdb/complaints.exp: breakpoint in captured_command_loop
...
there are two breakpoint locations instead of one.  This is due to
PR26096 - "gdb sets breakpoint at cold clone":
...
$ nm gdb | grep captured_command_loop| c++filt
0000000000659f20 t captured_command_loop()
00000000004230ed t captured_command_loop() [clone .cold]
...

Work around this by allowing multiple breakpoint locations for
captured_command_loop.

Reg-tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-29  Tom de Vries  <tdevries@suse.de>

	* lib/selftest-support.exp (selftest_setup): Allow breakpoint at
	multiple locations.