Fix dumping stack from test coredumps on Linux

[danmoseley]

• The dumps were in $EXECUTION_DIR. Pushd to that to find them.
• Corerun is someplace else. Use $RUNTIME_PATH/dotnet instead. It's what we actually ran, anyway.
• Add random suffix to keep more than one old dump

Tested locally by using kill -SIGABRT on running tests.

Fixes https://github.com/dotnet/corefx/issues/27818

[stephentoub]

Now we wait...

[stephentoub]

Initial results:
dotnet/corefx#27899 (comment)

Looks like we need gdb installed onto several of the images, and core dumps enabled on others. But RedHat and CentOS got the expected results :)