Avoid inheriting stderr in SelectiveExecutionTests.scala to prevent…

… flakiness from leaked processes (#4581)

Currently, the inherited `stderr` can remain open even as the spawned
processes are killed, causing the test to hang on waiting for the
`stderr` to close. This avoids inheriting stderr, so once the spawned
processes dies the test can terminate cleanly

integration/invalidation/selective-execution/src/SelectiveExecutionTests.scala

@@ -268,7 +268,7 @@ object SelectiveExecutionWatchTests extends UtestIntegrationTestSuite {
               ("--watch", "{foo.fooCommand,bar.barCommand}"),
               check = true,
               stdout = os.ProcessOutput.Readlines { line => output0 = output0 :+ line },
-              stderr = os.Inherit
+              stderr = os.ProcessOutput.Readlines { line => System.err.println(line) }
             )
           }