OpenBSD does not have native backtrace support.

There is libexecinfo as a 3rd-party package, but don't expect it be
available for now.

While we are doing this, introduce the necessary boilerplate to mark
tests as expected failures.

Author: 3405691582

Tests/Foundation/Tests/TestThread.swift

@@ -160,8 +160,16 @@ class TestThread : XCTestCase {
             ("test_currentThread", test_currentThread),
             ("test_threadStart", test_threadStart),
             ("test_mainThread", test_mainThread),
-            ("test_callStackSymbols", testExpectedToFailOnAndroid(test_callStackSymbols, "Android doesn't support backtraces at the moment.")),
-            ("test_callStackReturnAddresses", testExpectedToFailOnAndroid(test_callStackReturnAddresses, "Android doesn't support backtraces at the moment.")),
+            ("test_callStackSymbols", testExpectedToFailOnOpenBSD(
+		    testExpectedToFailOnAndroid(
+		        test_callStackSymbols,
+		    "Android doesn't support backtraces at the moment."),
+		"And not currently on OpenBSD.")),
+            ("test_callStackReturnAddresses", testExpectedToFailOnOpenBSD(
+                    testExpectedToFailOnAndroid(
+			test_callStackReturnAddresses,
+                    "Android doesn't support backtraces at the moment."),
+		"And not currently on OpenBSD.")),
             ("test_sleepForTimeInterval", test_sleepForTimeInterval),
             ("test_sleepUntilDate", test_sleepUntilDate),
             ("test_threadName", test_threadName),

Tests/Foundation/Utilities.swift

@@ -557,6 +557,14 @@ func shouldAttemptAndroidXFailTests(_ reason: String) -> Bool {
     #endif
 }
 
+func shouldAttemptOpenBSDXFailTests(_ reason: String) -> Bool {
+    #if os(OpenBSD)
+    return shouldAttemptXFailTests(reason)
+    #else
+    return true
+    #endif
+}
+
 #if !DARWIN_COMPATIBILITY_TESTS
 func testCaseExpectedToFail<T: XCTestCase>(_ allTests: [(String, (T) -> () throws -> Void)], _ reason: String) -> XCTestCaseEntry {
     return testCase(allTests.map { ($0.0, testExpectedToFail($0.1, "This test suite is disabled: \(reason)")) })
@@ -581,6 +589,10 @@ func testExpectedToFailOnAndroid<T>(_ test: @escaping (T) -> () throws -> Void,
     testExpectedToFailWithCheck(check: shouldAttemptAndroidXFailTests(_:), test, reason)
 }
 
+func testExpectedToFailOnOpenBSD<T>(_ test: @escaping (T) -> () throws -> Void, _ reason: String) -> (T) -> () throws -> Void {
+    testExpectedToFailWithCheck(check: shouldAttemptOpenBSDXFailTests(_:), test, reason)
+}
+
 func testExpectedToFailWithCheck<T>(check: (String) -> Bool, _ test:  @escaping (T) -> () throws -> Void, _ reason: String) -> (T) -> () throws -> Void {
     if check(reason) {
         return test