fix flaky test that can hang in CI (#205)

motivation: stable CI

background:
* testSignal can hang *sometimes* when the child processes it creates do not recieve the expeted signal.
* this has been observed when waitForFile timed-out and the test existed before issuing the signal.
* this behavior is unexpected and points to potential issue with xctest, but this fix attempts to workaround the underlying issue by making sure the sub-rocesses are forced-killed by the test itself

changes:
* regsiter the sub-processes in a process-set and force temrinate them at the end of the test

rdar://74356445

Author: tomerd

Tests/TSCBasicTests/ProcessTests.swift

@@ -116,6 +116,7 @@ class ProcessTests: XCTestCase {
 
   #if !os(Windows) // Signals are not supported in Windows
     func testSignals() throws {
+        let processes  = ProcessSet()
         let group = DispatchGroup()
 
         DispatchQueue.global().async(group: group) {
@@ -125,6 +126,7 @@ class ProcessTests: XCTestCase {
                     let file = tmpdir.appending(component: "pidfile")
                     let waitFile = tmpdir.appending(component: "waitFile")
                     let process = Process(args: self.script("print-pid"), file.pathString, waitFile.pathString)
+                    try processes.add(process)
                     try process.launch()
                     guard waitForFile(waitFile) else {
                         return XCTFail("Couldn't launch the process")
@@ -152,6 +154,7 @@ class ProcessTests: XCTestCase {
                     let file = tmpdir.appending(component: "pidfile")
                     let waitFile = tmpdir.appending(component: "waitFile")
                     let process = Process(args: self.script("subprocess"), file.pathString, waitFile.pathString)
+                    try processes.add(process)
                     try process.launch()
                     guard waitForFile(waitFile) else {
                         return XCTFail("Couldn't launch the process")
@@ -183,6 +186,9 @@ class ProcessTests: XCTestCase {
         if case .timedOut = group.wait(timeout: .now() + 10) {
             XCTFail("timeout waiting for signals to be processed")
         }
+
+        // rdar://74356445: make sure the processes are terminated as they *sometimes* cause xctest to hang
+        processes.terminate()
     }
   #endif