[gdb/testsuite] Add gdb.base/valgrind-infcall-2.exp

[gdb/testsuite] Add gdb.base/valgrind-infcall-2.exp

In commit ee3c5f8968 "Fix GDB crash when registers cannot be modified", we
fix a GDB crash:
...
$ valgrind /usr/bin/sleep 10000
==31595== Memcheck, a memory error detector
==31595== Command: /usr/bin/sleep 10000
==31595==
$ gdb /usr/bin/sleep
(gdb) target remote | vgdb --pid=31595
Remote debugging using | vgdb --pid=31595
  ...
$hex in __GI___nanosleep () at nanosleep.c:27
27	  return SYSCALL_CANCEL (nanosleep, requested_time, remaining);
(gdb) p printf ("bla")
terminate called after throwing an instance of 'gdb_exception_error'
Aborted (core dumped)
...

This patch adds a test-case for it.

Unfortunately, I was not able to trigger the error condition using a regular
vgdb_start, so I've added a parameter active_at_startup, and when set to 0
this causes valgrind to be started without --vgdb-error=0.

Tested on x86_64-linux.

Tested with the commit mentioned above reverted, resulting in:
...
(gdb) p printf ("bla")^M
terminate called after throwing an instance of 'gdb_exception_error'^M
ERROR: GDB process no longer exists
GDB process exited with wait status 6152 exp10 0 0 CHILDKILLED SIGABRT SIGABRT
UNRESOLVED: gdb.base/valgrind-infcall-2.exp: do printf
...

gdb/testsuite/ChangeLog:

2020-07-17  Tom de Vries  <tdevries@suse.de>

	* gdb.base/valgrind-infcall-2.c: New test.
	* gdb.base/valgrind-infcall-2.exp: New file.
	* lib/valgrind.exp (vgdb_start): Add and handle active_at_startup.