Linux ELF core tests: kernel.core_use_pid also affects core filename

Two of the tests (LinuxCoreDumperTest.VerifyDumpWithMultipleThreads and
ElfCoreDumpTest.ValidCoreFile) would fail if the sysctl variable
kernel.core_use_pid was set to 1, because they depend on the filename of a
native core dump being "core" (not "core.%d" with the pid).  There's already a
check for the more general kernel.core_pattern facility which skips the test if
the name is changed, so this patch adds core_use_pid to it.

I notice that there doesn't seem to be any visible warning if the test is
skipped for this reason.  Should I try to fix that?  (And, if so, pointers to
how to make that happen?)