Always set the XCTest bundle path on Darwin when running exit tests. (#422)

Xcode is a complicated beast and there are code paths that lead to us
running a test hosted by XCTest that isn't discoverable by the existing
exit test implementation. This PR makes that implementation more robust
in these scenarios and fixes spurious exit test failures that were
resulting.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitTest.swift

@@ -235,18 +235,42 @@ extension ExitTest {
     // We only need to pass arguments when hosted by XCTest.
     let childArguments: [String] = {
       var result = [String]()
-      if let xcTestCaseIdentifier {
+      lazy var xctestTargetPath = Environment.variable(named: "XCTestBundlePath")
+        ?? CommandLine.arguments().dropFirst().last
 #if SWT_TARGET_OS_APPLE
-        result += ["-XCTest", xcTestCaseIdentifier]
+      // If the running executable appears to be the XCTest runner executable in
+      // Xcode, figure out the path to the running XCTest bundle. If we can find
+      // it, then we can re-run the host XCTestCase instance.
+      // TODO: use basename_r() instead of parsing the path.
+      var isHostedByXCTest = false
+      if let executablePath = try? childProcessExecutablePath.get() {
+        executablePath.withCString { childProcessExecutablePath in
+          withUnsafeTemporaryAllocation(of: CChar.self, capacity: strlen(childProcessExecutablePath) + 1) { baseName in
+            if nil != basename_r(childProcessExecutablePath, baseName.baseAddress!) {
+              isHostedByXCTest = 0 == strcmp(baseName.baseAddress!, "xctest")
+            }
+          }
+        }
+      }
+
+      if isHostedByXCTest, let xctestTargetPath {
+        // HACK: if the current test is being run from within Xcode, we don't
+        // always know we're being hosted by an XCTestCase instance. In cases
+        // where we don't, but the XCTest environment variable specifying the
+        // test bundle is set, assume we _are_ being hosted and specify a
+        // blank test identifier ("/") to force the xctest command-line tool
+        // to run.
+        let xcTestCaseIdentifier = xcTestCaseIdentifier ?? "/"
+        result += ["-XCTest", xcTestCaseIdentifier, xctestTargetPath]
+      }
 #else
+      if let xcTestCaseIdentifier {
         result.append(xcTestCaseIdentifier)
-#endif
-        if let xctestTargetPath = Environment.variable(named: "XCTestBundlePath") {
-          result.append(xctestTargetPath)
-        } else if let xctestTargetPath = CommandLine.arguments().last {
+        if let xctestTargetPath {
           result.append(xctestTargetPath)
         }
       }
+#endif
       return result
     }()
 

Sources/_TestingInternals/include/Includes.h

@@ -95,6 +95,10 @@
 #include <wasi/libc-environ.h>
 #endif
 
+#if __has_include(<libgen.h>)
+#include <libgen.h>
+#endif
+
 #if defined(_WIN32)
 #define WIN32_LEAN_AND_MEAN
 #define NOMINMAX