For corefx testing, use a matching corefx repo version git hash
For corefx testing, use a matching corefx repo version git hash

If the user doesn't specify a specific corefx commit hash to use,
try to find the matching commit hash in the coreclr repro. If the
matching hash can't be found, use 'HEAD'.

We find the matching corefx commit hash by first parsing file
'dependencies.props' at the root of the coreclr repro, looking for this:
   <MicrosoftPrivateCoreFxNETCoreAppPackageVersion>4.5.0-preview1-26112-01</MicrosoftPrivateCoreFxNETCoreAppPackageVersion>
This determines the corefx package version that matches. Next, we look for the version.txt
file in the package cache, e.g.,
   <coreclr_root>\packages\microsoft.private.corefx.netcoreapp\4.5.0-preview1-26112-01\version.txt
The contents of this file is exactly the git commit hash
we need to use, e.g.:
   197a0699b08087ea85581679afdd9fd7b5c465c3
The version.txt file is created when the corefx package is restored,
which happens when doing one of:
   Windows: tests\runtests.cmd GenerateLayoutOnly
   non-Windows: build-test.sh generatelayoutonly

It would also be possible to not depend on the package already
being downloaded, but instead download the correct package here,
using the determined "MicrosoftPrivateCoreFxNETCoreAppPackageVersion"
package version, e.g.:
   https://dotnet.myget.org/F/dotnet-core/api/v2/package/Microsoft.Private.CoreFx.NETCoreApp/4.5.0-preview1-26112-01
and then extracting the ZIP archive to find the version.txt file.

This might get easier if the corefx commit hash is added directly to dependencies.props, as
discussed in https://github.com/dotnet/buildtools/issues/1141.

wtgodbe/coreclr,yizhang82/coreclr,wtgodbe/coreclr,cshung/coreclr,poizan42/coreclr,yizhang82/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,krk/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,mmitche/coreclr,mmitche/coreclr,poizan42/coreclr,cshung/coreclr,mmitche/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,mmitche/coreclr,yizhang82/coreclr,krk/coreclr,poizan42/coreclr,wtgodbe/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,mmitche/coreclr,poizan42/coreclr,krk/coreclr,wtgodbe/coreclr,krk/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,poizan42/coreclr,krk/coreclr,krk/coreclr,mmitche/coreclr,poizan42/coreclr,wtgodbe/coreclr,ruben-ayrapetyan/coreclr