[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

---------

Co-authored-by: Max Desiatov <[email protected]>

Author: grynspan

Sources/Commands/SwiftTestCommand.swift

@@ -154,6 +154,21 @@ struct TestCommandOptions: ParsableArguments {
     var enableExperimentalTestOutput: Bool {
         return testOutput == .experimentalSummary
     }
+
+    /// Path where swift-testing's JSON configuration should be read.
+    @Option(name: .customLong("experimental-configuration-path"),
+            help: .hidden)
+    var configurationPath: AbsolutePath?
+
+    /// Path where swift-testing's JSON output should be written.
+    @Option(name: .customLong("experimental-event-stream-output"),
+            help: .hidden)
+    var eventStreamOutputPath: AbsolutePath?
+
+    /// The schema version of swift-testing's JSON input/output.
+    @Option(name: .customLong("experimental-event-stream-version"),
+            help: .hidden)
+    var eventStreamVersion: Int?
 }
 
 /// Tests filtering specifier, which is used to filter tests to run.
@@ -682,9 +697,10 @@ extension SwiftTestCommand {
                 sanitizers: globalOptions.build.sanitizers
             )
 
+            let additionalArguments = ["--list-tests"] + CommandLine.arguments.dropFirst()
             let runner = TestRunner(
                 bundlePaths: testProducts.map(\.binaryPath),
-                additionalArguments: ["--list-tests"],
+                additionalArguments: additionalArguments,
                 cancellator: swiftCommandState.cancellator,
                 toolchain: toolchain,
                 testEnv: testEnv,