Phobos tests sometimes run out of memory

[wilzbach]

posix.mak:364: recipe for target 'generated/linux/release/64/unittest/std/regex/internal/tests2.o' failed
--
  | make[1]: *** [generated/linux/release/64/unittest/std/regex/internal/tests2.o] Terminated
  | posix.mak:364: recipe for target 'generated/linux/release/64/unittest/std/regex/internal/tests.o' failed
  | make[1]: *** [generated/linux/release/64/unittest/std/regex/internal/tests.o] Terminated
  | posix.mak:312: recipe for target 'unittest-release' failed

(from https://buildkite.com/dlang/ci/builds/523#0cd96a29-34a3-487d-8e3e-4d614d08aa51)

We had similar problems with the auto-tester in the past, see e.g. dlang/phobos#6061