Merge dev15.9.x to master

Fix tests to be only conditionally skipped

Clean up and align targets with RepoToolset

Fix folder names

Fix the folder names in the myget publishing manifest

The xUnit framework appears to be reading the Skip property defined on
`ConditionalFactAttribute` vs. the one on `FactAttribute`. Even with an
unconditional throw this was interpreted as "not skipped" and hence all
tests ran.

When conditionally skipping tests for CoreClr I mistakenly
unconditionally skipped them in my attempt to document the reason for
the skip. The use of the Skip property causes the test to be
unconditionally skipped, not documenting the reason.

In addition to fixing this I prevented this from happening in the
future. It's very rare for a `[ConditionalFact]` to actually want to use
Skip as it means unconditionally skip. This is generally only for short
term bug work arounds. Hence I changed `ConditionalFactAttribute` as
follows:

- Added a new Skip property that is obsolete and has no setter.
Completely prevents the code `[ConditionalFact(..., Skip = )`
- Added a Reason property documentation in the case the test is skipped.
- Added an AlwaysSkip property in the case you always want to skip.

Misc infrastructure fixes

Merge dev15.8.x to dev15.9.x

Fixes the following issues in the determinism tests:

1. Use `subst` instead of `mklink` as the former doesn't require
administrator rights
2. Put the determinism.zip file into the Binaries\Log directory
3. Include the key files in determinism.zip

The existing code for creating a Mutex for the current user failed when
the user name included a `$`. The reasons for why it failed are unknown
but verified using the WindowsIdentity instance directly accounts for
this.

Merge dev15.9.x to master

Merge dev15.8.x to dev15.9.x

Set VSSDKTargetPlatformRegRootSuffix for Roslyn.csproj

The great project rename

Remove duplication of command group/id constants

Fixes #26028

Add decompiler options to FeatureOnOffOptionsProvider

Use correct temporary folder for solutions created in integration tests

Use a 75 minute test timeout on CI

Closes #28410

Merge dev15.8.x to dev15.9.x

Add a NuGet.Config to stabilize tests