[gdb/testsuite] Fix timeout in gdb.base/valgrind-infcall-2.exp

Since commit 6d5702a5 "Fix test case gdb.base/valgrind-bt.exp" I run into:
...
FAIL: gdb.base/valgrind-infcall-2.exp: target remote for vgdb (timeout)
FAIL: gdb.base/valgrind-infcall-2.exp: monitor v.set gdb_output (timeout)
...

The commit adds this line in proc vgdb_start:
...
    set vgdbcmd "set remotetimeout 3"
...
which has no effect given that the value of var vgdbcmd is not used before
it's overwritten.  We can fix this by doing instead:
...
    set_remotetimeout 3
...

The FAIL I'm observing is fixed by increasing the remotetimeout value to 4.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-04-29  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27786
	* lib/valgrind.exp (vgdb_start): Use set_remotetimeout.  Increase
	remotetimeout to 4.