Add tests for Basics/AsyncProcess (swiftlang#7824)

`AsyncProcess` replicates `TSCBasic.Process` with additions that make it
easier to use with Swift Concurrency. To date we didn't replicate the
test suite for it, this PR aims to fix that omission.

Author: MaxDesiatov

Package.swift

@@ -619,6 +619,11 @@ let package = Package(
                 "Archiver/Inputs/archive.zip",
                 "Archiver/Inputs/invalid_archive.tar.gz",
                 "Archiver/Inputs/invalid_archive.zip",
+                "processInputs/long-stdout-stderr",
+                "processInputs/exit4",
+                "processInputs/simple-stdout-stderr",
+                "processInputs/deadlock-if-blocking-io",
+                "processInputs/in-to-out",
             ]
         ),
         .testTarget(

Sources/Basics/AsyncProcess.swift

@@ -173,6 +173,8 @@ package final class AsyncProcess {
         case stdinUnavailable
     }
 
+    package typealias OutputStream = AsyncStream<[UInt8]>
+
     package enum OutputRedirection {
         /// Do not redirect the output
         case none

Sources/Basics/Environment/Environment.swift

@@ -26,7 +26,7 @@ import Darwin.C
 #endif
 
 // FIXME: Use Synchronization.Mutex when available
-class Mutex<T>: @unchecked Sendable {
+private final class Mutex<T>: @unchecked Sendable {
     var lock: NSLock
     var value: T
 
@@ -118,7 +118,7 @@ extension Environment {
 // MARK: - Global Environment
 
 extension Environment {
-    static let _cachedCurrent = Mutex<Self?>(value: nil)
+    fileprivate static let _cachedCurrent = Mutex<Self?>(value: nil)
 
     /// Vends a copy of the current process's environment variables.
     ///