Tests: mitigate progressivity issue in LegacyHTTPClientTests (#7011)

The legacy client can block several threads by semaphore and the test
case itself blocks a thread per request. The test case waits until all
requests go in the middle of the process, but some of requests won't be
started due to a lack of available threads on a machine with a few
cores. Reducing the number of concurrent requests does not solve the
root issue (it ultimately requires lock/wait free implementation and I
think it's not worth it just for legacy one), but should be enough to
repair CI.

Author: kateinoigakukun

Tests/BasicsTests/LegacyHTTPClientTests.swift

@@ -596,7 +596,7 @@ final class LegacyHTTPClientTests: XCTestCase {
         let observability = ObservabilitySystem.makeForTesting()
         let cancellator = Cancellator(observabilityScope: observability.topScope)
 
-        let total = 10
+        let total = min(10, ProcessInfo.processInfo.activeProcessorCount / 2)
         // this DispatchGroup is used to wait for the requests to start before calling cancel
         let startGroup = DispatchGroup()
         // this DispatchGroup is used to park the delayed threads that would be cancelled