Tests: Fix -Wunreachable-code warnings from clang

Tests/Kernel/elf-execve-mmap-race.cpp

@@ -123,7 +123,6 @@ int main()
                 hax = !hax;
                 usleep(1);
             }
-            return nullptr;
         },
         &writable_program_headers[0]);
 
@@ -139,5 +138,4 @@ int main()
         printf("waitpid\n");
         waitpid(-1, nullptr, 0);
     }
-    return 0;
 }

Tests/Kernel/nanosleep-race-outbuf-munmap.cpp

@@ -55,7 +55,6 @@ static void* yanker_fn(void* shared_)
     if (child_pid < 0) {
         printf("Yanker: Fork failed: %d\n", child_pid);
         pthread_exit(nullptr); // See below
-        return nullptr;
     }
 
     if (child_pid > 0) {
@@ -63,7 +62,6 @@ static void* yanker_fn(void* shared_)
         // FIXME: LibPthread bug: returning during normal operation causes nullptr deref.
         // Workaround: Exit manually.
         pthread_exit(nullptr);
-        return nullptr;
     }
 
     // Give parent *thread* a moment to die.

Tests/Kernel/null-deref-close-during-select.cpp

@@ -22,7 +22,6 @@ int main(int, char**)
             printf("ST: close()\n");
             close(pipefds[1]);
             pthread_exit(nullptr);
-            return nullptr;
         },
         nullptr);
 

Tests/Kernel/path-resolution-race.cpp

@@ -18,5 +18,4 @@ int main()
     for (;;) {
         chdir("/tmp/x");
     }
-    return 0;
 }

Tests/Kernel/uaf-close-while-blocked-in-read.cpp

@@ -23,7 +23,6 @@ int main(int, char**)
             close(pipefds[0]);
             close(pipefds[1]);
             pthread_exit(nullptr);
-            return nullptr;
         },
         nullptr);