Skip testMakeStreamOnShutdownSubchannel on 5.8 and earlier (#1885)

Motivation:

`testMakeStreamOnShutdownSubchannel` occasionally hits an assertion
failure in pthread caused by a bug in the Swift concurrency runtime.
This was fixed in 5.9.

Modifications:

- Skip the test for older compiler versions

Result:

Fewer false negatives

Tests/GRPCHTTP2CoreTests/Client/Connection/LoadBalancers/SubchannelTests.swift

@@ -39,6 +39,10 @@ final class SubchannelTests: XCTestCase {
   }
 
   func testMakeStreamOnShutdownSubchannel() async throws {
+    #if compiler(<5.9)
+    throw XCTSkip("Occasionally crashes due to a Swift 5.8 concurrency runtime bug")
+    #endif
+
     let subchannel = self.makeSubchannel(
       address: .unixDomainSocket(path: "ignored"),
       connector: .never