Fix build script for XCTest so it can link _TestingInterop
This is follow up for #87318, where I updated the build script but still left a
few shortcomings that are addressed here.
I've validated these changes more extensively by building in CI against the
(currently) draft PR in swift-corelibs-xctest that introduces the
_TestingInterop linkage requirement.
* Update the cmake search path SwiftTesting_DIR (swift_testing -> swifttesting)
Failed to find SwiftTesting in the earlier build: https://ci.swift.org/job/swift-corelibs-xctest-PR-Linux/139/consoleText
```
-DSwiftTesting_DIR=/home/build-user/build/Ninja-ReleaseAssert/swift_testing-linux-x86_64/cmake/modules
...
[2026-02-23T23:30:38.885Z] -- Could NOT find SwiftTesting (missing: SwiftTesting_DIR)
```
But it was created earlier as swifttesting without the underscore:
```
[2026-02-23T23:29:27.132Z] + mkdir -p /home/build-user/build/Ninja-ReleaseAssert/swifttesting-linux-x86_64
```
* Update build.ps1 for linking XCTest against _TestingInterop on Windows too
* Test-XCTest: include Testing in `$env:Path` so it can find the interop dll
Note that we provide `$(Get-ProjectBinaryCache $BuildPlatform
Testing)**\bin**` (emphasis added) because of this setting in Testing's
cmake config.
```
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
```
* Fix build ordering: put Testing before XCTest
* Specify SwiftTesting_DIR for cmake
c802acf6f6