[gdb/testsuite] Factor out lib/valgrind.exp

[gdb/testsuite] Factor out lib/valgrind.exp

Factor out common code related to vgdb setup and cleanup in valgrind-bt.exp,
valgrind-disp-step.exp and gdb.base/valgrind-infcall.exp.

Tested on x86_64-linux with and without --target_board=native-gdbserver.

2018-10-31  Tom de Vries  <tdevries@suse.de>

	* lib/valgrind.exp: New file.
	 (vgdb_start, vgdb_stop): New procs, factored out of ...
	* gdb.base/valgrind-bt.exp: ... here, ...
	* gdb.base/valgrind-disp-step.exp: ... here and ...
	* gdb.base/valgrind-infcall.exp: ... here.