Pre-consolidation mop up fixes dotnet/coreclr#3 (dotnet/coreclr#27836)

Pre-consolidation mop up fixes dotnet/coreclr#3 (dotnet/coreclr#27836)

This change contains the remaining fixes I made while digging
through the PR run in the runtime repo:

1) Some more repo-relative vs. coreclr-relative repo adjustments
reflecting the migration process;

2) Make sure that XunitTestBinBase always ends with a directory
separator, otherwise my recent fix for Common folder exclusion
doesn't work;

3) Opportunistically shorten project names of two native interop
test components - as the project name is repeated about 3-4 times
in some intermediate paths, it quickly exceeds the standard
Windows path length limit. Please note there's no functional
change in the test.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/35e54e804f48ccb8ec4b852fa67a9f88850d214b