[cxx-interop] Propagate interop flag to the test entry point target

### Motivation:

Projects that use C++ interop for some of the Swift targets were failing
to build in test mode (`swift test`) because the generated test entry
point target wasn't being compiled with C++ interop enabled.

### Modifications:

The logic that propagates the C++ interop compiler flag
(`-cxx-interoperability-mode`) to the discovery target now also runs for
the entry point target.



rdar://125498997

Author: egorzhdan

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -555,11 +555,11 @@ package final class SwiftTargetBuildDescription {
             args += ["-color-diagnostics"]
         }
 
-        // If this is a generated test discovery target, it might import a test
+        // If this is a generated test discovery target or a test entry point, it might import a test
         // target that is built with C++ interop enabled. In that case, the test
         // discovery target must enable C++ interop as well
         switch testTargetRole {
-        case .discovery:
+        case .discovery, .entryPoint:
             for dependency in try self.target.recursiveTargetDependencies() {
                 let dependencyScope = self.buildParameters.createScope(for: dependency)
                 let dependencySwiftFlags = dependencyScope.evaluate(.OTHER_SWIFT_FLAGS)

Tests/BuildTests/BuildPlanTests.swift

@@ -4063,6 +4063,9 @@ final class BuildPlanTests: XCTestCase {
 
             let testDiscovery = try result.target(for: "APackageDiscoveredTests").swiftTarget().compileArguments()
             XCTAssertMatch(testDiscovery, [.anySequence, "-cxx-interoperability-mode=default", "-Xcc", "-std=c++17"])
+
+            let testEntryPoint = try result.target(for: "APackageTests").swiftTarget().compileArguments()
+            XCTAssertMatch(testEntryPoint, [.anySequence, "-cxx-interoperability-mode=default", "-Xcc", "-std=c++17"])
         }
 
         // omit frame pointers explicitly set to true