Apply Pester.BeforeContainer.ps1 when only discovery runs

[nohwnd]

Fix #3008

Run.SkipRun returns the discovered tree without executing anything, which is the path the VS Code Test Explorer uses to populate. It goes through Discover-Test, a batch discovery over all containers, and that function never took the BeforeContainer map, so it called Invoke-ContainerDiscovery without -BeforeContainerFile. A file whose discovery depends on the setup discovered fine in a normal run and came back empty when only discovery was requested.

# <root>/Pester.BeforeContainer.ps1
BeforeDiscovery { $script:Cases = 1..3 }

# <root>/tests/Some.Tests.ps1
Describe 'd' {
    It 'case <_>' -ForEach $Cases { }
}

6.1.0, Run.SkipRun          discovered 0
this branch, Run.SkipRun    discovered 3
this branch, normal run     discovered 3

Discover-Test now takes the same map the interleaved discover -> run path gets, and resolves it per container, because which files apply depends on where the container is after #2993. Find-Test forwards it too.

The per container lookup was written inline in Invoke-Test, and is a function now that both paths call, so they cannot drift apart again. That also removes the case-insensitive variable name trap the inline version needed a comment to explain.

Verification

tst/Pester.RSpec.Parallel.ts.ps1 45/45 in the cascade block, whole file green. tst/Pester.Runtime.ts.ps1 and tst/Pester.RSpec.ts.ps1 green as well, since this touches shared discovery code.

Added a test next to the other cascade tests that asserts Run.SkipRun and a normal run find the same tests.

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