FoundationEssentialTests: nanosleep is not portable, avoid it

Use `Thread.sleep(forTimeInterval:)` instead of `nanosleep` which is not
a portable sleep primitive. This allows us to build the package tests on
Windows.

Author: compnerd

Tests/FoundationEssentialsTests/ProcessInfoTests.swift

@@ -99,8 +99,7 @@ final class ProcessInfoTests : XCTestCase {
         XCTAssertTrue(
             now > 1, "ProcessInfo returned an unrealistically low system uptime")
         // Sleep for 0.1s
-        var ts: timespec = timespec(tv_sec: 0, tv_nsec: 100000000)
-        nanosleep(&ts, nil)
+        Task.sleep(for: .milliseconds(1000))
         XCTAssertTrue(
             ProcessInfo.processInfo.systemUptime > now,
             "ProcessInfo returned the same system uptime with 400")