Fix issue with running test on windows w/no-parallel

The AsyncProcess Tests were using a TSCTestSupport method to temporarly change the PATH env but seems to have a bug where it removes the variable instead of restoring resulting in any future test needing PATH to fail.  This switch it to using the same method that is in SwiftPM Environment.swft

Author: daveinglis

Tests/BasicsTests/AsyncProcessTests.swift

@@ -20,7 +20,6 @@ import struct TSCBasic.ByteString
 import struct TSCBasic.Format
 import class TSCBasic.Thread
 import func TSCBasic.withTemporaryFile
-import func TSCTestSupport.withCustomEnv
 
 #if os(Windows)
 let catExecutable = "type"
@@ -168,7 +167,7 @@ final class AsyncProcessTests: XCTestCase {
             #endif
             try localFileSystem.writeFileContents(tempExecutable, bytes: exitScriptContent)
 
-            try withCustomEnv(["PATH": tmpdir.pathString]) {
+            try Environment.makeCustom(["PATH": tmpdir.pathString]) {
                 XCTAssertEqual(AsyncProcess.findExecutable("nonExecutableProgram"), nil)
             }
         }
@@ -184,7 +183,7 @@ final class AsyncProcessTests: XCTestCase {
 
             """)
 
-            try withCustomEnv(["PATH": tmpdir.pathString]) {
+            try Environment.makeCustom(["PATH": tmpdir.pathString]) {
                 do {
                     let process = AsyncProcess(args: "nonExecutableProgram")
                     try process.launch()

Tests/BasicsTests/Environment/EnvironmentTests.swift

@@ -16,6 +16,7 @@ import Foundation
 import Basics
 
 import Testing
+import _InternalTestSupport
 
 struct EnvironmentTests {
     @Test
@@ -144,6 +145,20 @@ struct EnvironmentTests {
         #expect(Environment.current[key] == nil)
     }
 
+    /// Important: This test is inherently race-prone, if it is proven to be
+    /// flaky, it should run in a singled threaded environment/removed entirely.
+    @Test
+    func makeCustomPathEnv() async throws {
+        try XCTSkipOnWindows(because: "Fails in CI due to parallel running tests", skipPlatformCi: true, skipSelfHostedCI: true)
+        let customEnvironment: Environment = .current
+        let origPath = customEnvironment[.path]
+
+        try Environment.makeCustom(["PATH": "/foo/bar"]) {
+            #expect(Environment.current[.path] == "/foo/bar")
+        }
+        #expect(Environment.current[.path] == origPath)
+    }
+
     /// Important: This test is inherently race-prone, if it is proven to be
     /// flaky, it should run in a singled threaded environment/removed entirely.
     @Test

Tests/FunctionalTests/TraitTests.swift

@@ -328,8 +328,7 @@ final class TraitTests: XCTestCase {
                         error: Disabled default traits by package 'disablingemptydefaultsexample' on package 'Package11' that declares no traits. This is prohibited to allow packages to adopt traits initially without causing an API break.
                         
                         """
-                XCTAssertTrue(stderr.contains(expectedErr))
-
+                XCTAssertMatch(stderr, .contains(expectedErr))
             }
         }
     }