[6.0] Pass through experimental swift-testing flags (#7564)
Explanation: Pass through `--experimental-configuration-path`,
`--experimental-event-stream-output`, and
`--experimental-event-stream-version` as used by swift-testing. These
flags are needed for continued experimental support of swift-testing
when using the Swift 6 toolchain.
Scope: Hidden flags in `swift test`.
Original PR: #7534, #7551
Risk: No obvious risk. These flags have no effect on `swift test` itself
and are simply passed through verbatim to the swift-testing test
executable.
Testing: Tested at-desk that the flags are seen by swift-testing with an
appropriate toolchain.
Reviewer: @bnbarham, @MaxDesiatov, @stmontgomery
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Co-authored-by: Max Desiatov <m_desiatov@apple.com>