tests: mount proc before `exec valgrind` (#4044)

tests: mount proc before `exec valgrind` (#4044)

Fixes:
$ sudo make run QEMU_BIN=/usr/bin/qemu-system-x86_64  KERNEL_APPEND=systemd.unit=multi-user.target SKIP_INITRD=yes
TEST RUN: Basic systemd setup
+ /usr/bin/qemu-system-x86_64 -smp 1 -net none -m 512M -nographic -kernel /boot/vmlinuz-4.6.7-200.fc23.x86_64 -drive format=raw,cache=unsafe,file=/var/tmp/systemd-test.izx99J/rootdisk.img -append 'root=/dev/sda1 raid=noautodetect loglevel=2 init=/usr/lib/systemd/systemd-under-valgrind rw console=ttyS0 selinux=0 systemd.unified_cgroup_hierarchy=no systemd.unit=multi-user.target '
valgrind: warning (non-fatal): readlink("/proc/self/exe") failed.
valgrind: continuing, however --trace-children=yes will not work.
--1:0: aspacem Valgrind: FATAL: can't open /proc/self/maps
--1:0: aspacem Exiting now.
[    3.152367] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
[    3.152367]
[    3.152367] CPU: 0 PID: 1 Comm: memcheck-amd64- Not tainted 4.6.7-200.fc23.x86_64 #1
[    3.152367] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150714_191134- 04/01/2014
[    3.152367]  0000000000000086 0000000047ca1666 ffff88001ea43e00 ffffffff813d954e
[    3.152367]  ffffffff81a205a0 ffff88001ea43e98 ffff88001ea43e88 ffffffff811b5557
[    3.152367]  ffffffff00000010 ffff88001ea43e98 ffff88001ea43e30 0000000047ca1666
[    3.152367] Call Trace:
[    3.152367]  [<ffffffff813d954e>] dump_stack+0x63/0x85
[    3.152367]  [<ffffffff811b5557>] panic+0xde/0x220
[    3.152367]  [<ffffffff810ab9c3>] do_exit+0xb43/0xb50
[    3.152367]  [<ffffffff810aba57>] do_group_exit+0x47/0xb0
[    3.152367]  [<ffffffff810abad4>] SyS_exit_group+0x14/0x20
[    3.152367]  [<ffffffff817dae72>] entry_SYSCALL_64_fastpath+0x1a/0xa4
[    3.152367] Kernel Offset: disabled
[    3.152367] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
[    3.152367]
QEMU: Terminated