test: pin stress-ng --vm-method to a portable scalar method in TEST-55-OOMD

test: pin stress-ng --vm-method to a portable scalar method in TEST-55-OOMD

The stress-ng "vm" stressor's default --vm-method=all cycles through every
VM stress method, including newer ones that use AVX-512 instructions. On
CPUs without AVX-512 support (e.g. AMD Zen 1 to 3) those methods crash with
SIGILL. In testcase_oom_rulesets_lasting_sec all 10 stress-ng workers die
within ~2.34 seconds, so by the time the 6 second sleep elapses the unit
is already in failed/exit-code state and the assert_eq for
ActiveState=active trips.

Pin --vm-method=zero-one, a long-standing scalar method, on all four
stress-ng --vm invocations in this test (the two transient services in
testcase_oom_rulesets and testcase_oom_rulesets_lasting_sec, plus
TEST-55-OOMD-testbloat.service and TEST-55-OOMD-testmunch.service) so the
workers do not crash on AVX-512-less CPUs. testbloat, testmunch and
testcase_oom_rulesets have not been observed failing because they get
OOM-killed by systemd-oomd within ~1 to 2 seconds, before stress-ng cycles
into an AVX-512 method, but they share the same latent flake.

Journal excerpts from the failing run, TEST-55-OOMD-slowrule.service in
testcase_oom_rulesets_lasting_sec (journalctl -o short-monotonic):

[   58.018676] stress-ng[1015]: invoked with '/usr/bin/stress-ng --timeout 15s --vm 10 --vm-bytes 50M --vm-keep' by user 0 'root'
[   59.866072] stress-ng[1030]: stress-ng: debug: [1030] caught SIGILL, address 0x000055bd8d609140 (ILL_ILLOPN)
[   59.921050] stress-ng[1030]: stress-ng: debug: [1030] stress-ng: info: 0x000055bd8d609140:<62>71 fd 48 6f 2d 36 14 1c 00 c5 d1 ef ed 49 29
[   59.929310] stress-ng[1015]: stress-ng: error: [1015] vm: [1021] terminated with an error, exit status=2 (stressor failed)
[   60.364111] stress-ng[1015]: stress-ng: info:  [1015] failed: 10: vm (10)
[   60.364493] stress-ng[1015]: stress-ng: info:  [1015] unsuccessful run completed in 2.34 secs
[   60.371290] systemd[1]: TEST-55-OOMD-slowrule.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
[   60.371396] systemd[1]: TEST-55-OOMD-slowrule.service: Failed with result 'exit-code'.
[   64.017061] TEST-55-OOMD.sh[1010]: + assert_eq failed active
[   64.018167] TEST-55-OOMD.sh[1039]: FAIL: expected: 'active' actual: 'failed'

The faulting bytes marked by stress-ng with <62> (the byte at the
instruction pointer) decode unambiguously to an AVX-512 VMOVDQA64 using
the 512-bit zmm13 register, confirmed independently by two disassemblers:

  $ printf '\x62\x71\xfd\x48\x6f\x2d\x36\x14\x1c\x00' | ndisasm -b 64 -
  00000000  6271FD486F2D3614  vmovdqa64 zmm13,zword [rel 0x1c1440]
           -1C00

  $ echo '0x62, 0x71, 0xfd, 0x48, 0x6f, 0x2d, 0x36, 0x14, 0x1c, 0x00' | \
        llvm-mc -disassemble -triple=x86_64 -mattr=+avx512f
          .text
          vmovdqa64       1840182(%rip), %zmm13

The leading 0x62 is the EVEX prefix (exclusive to AVX-512 on this target),
zmm13 is a 512-bit register that only exists when AVX-512 is implemented,
and VMOVDQA64 requires the AVX512F (Foundation) CPUID feature (Intel SDM
Vol 2C). Executing this on a CPU without AVX-512 raises #UD, delivered by
the kernel as SIGILL/ILL_ILLOPN, matching the journal entry above. The
same journal shows the kernel reporting "kvm_amd: TSC scaling supported",
i.e. the guest is on AMD KVM, and AMD did not ship AVX-512 before Zen 4.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>