Fixes warnings in SwiftTestingRunner.swift (#1565)

Fixes #1557.

Before:
```sh
> bazel build //tools/test_observer
INFO: Analyzed target //tools/test_observer:test_observer (0 packages loaded, 0 targets configured).
INFO: From Compiling Swift module //tools/test_observer:test_observer:
tools/test_observer/SwiftTestingRunner.swift:83:26: warning: result of call to 'withLock' is unused
 81 |       switch testOrSuite {
 82 |       case .suite(let suiteID):
 83 |         discoveredSuites.withLock { $0.insert(suiteID) }
    |                          `- warning: result of call to 'withLock' is unused
 84 |       case .test(let test):
 85 |         collector.addTest(test)

tools/test_observer/SwiftTestingRunner.swift:179:34: warning: trailing closure in this context is confusable with the body of the statement; pass as a parenthesized argument to silence this warning
177 |       case .string(let testID) = payload["testID"],
178 |       // Ignore suites. The xUnit recorder reconstructs the hierarchy.
179 |       !discoveredSuites.withLock { $0.contains(testID) },
    |                                  `- warning: trailing closure in this context is confusable with the body of the statement; pass as a parenthesized argument to silence this warning
180 |       case .object(let instantJSON) = payload["instant"],
181 |       case .number(let absolute) = instantJSON["absolute"]
INFO: Found 1 target...
Target //tools/test_observer:test_observer up-to-date:
  bazel-bin/tools/test_observer/BazelTestObservation.swiftdoc
  bazel-bin/tools/test_observer/BazelTestObservation.swiftmodule
  bazel-bin/tools/test_observer/libtest_observer.a
INFO: Elapsed time: 4.960s, Critical Path: 4.85s
INFO: 3 processes: 1 internal, 1 darwin-sandbox, 1 worker.
INFO: Build completed successfully, 3 total actions
```

After:
```sh
> bazel build //tools/test_observer
INFO: Analyzed target //tools/test_observer:test_observer (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //tools/test_observer:test_observer up-to-date:
  bazel-bin/tools/test_observer/BazelTestObservation.swiftdoc
  bazel-bin/tools/test_observer/BazelTestObservation.swiftmodule
  bazel-bin/tools/test_observer/libtest_observer.a
INFO: Elapsed time: 0.115s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
```

Author: csmulhern

tools/test_observer/Locked.swift

@@ -60,6 +60,7 @@ public struct Locked<Value>: Sendable where Value: Sendable {
   ///
   /// Upon acquiring the lock, the body is passed a mutable copy of the value, which it has
   /// exclusive access to for the duration of the body. Mutations will affect the underlying value.
+  @discardableResult
   public nonmutating func withLock<Result>(
     _ body: (inout Value) throws -> Result
   ) rethrows -> Result {

tools/test_observer/SwiftTestingRunner.swift

@@ -176,7 +176,7 @@ public final class SwiftTestingRunner: Sendable {
       case .string(let kind) = payload["kind"],
       case .string(let testID) = payload["testID"],
       // Ignore suites. The xUnit recorder reconstructs the hierarchy.
-      !discoveredSuites.withLock { $0.contains(testID) },
+      !discoveredSuites.withLock({ $0.contains(testID) }),
       case .object(let instantJSON) = payload["instant"],
       case .number(let absolute) = instantJSON["absolute"]
     else {