FailingAllocator: Only capture the stack trace of the first induced allocation failure

This is a precaution to avoid confusing stack traces on the off chance that FailingAllocator continues to try to allocate after the first failure.

lib/std/testing/failing_allocator.zig

@@ -56,6 +56,7 @@ pub const FailingAllocator = struct {
         return_address: usize,
     ) error{OutOfMemory}![]u8 {
         if (self.index == self.fail_index) {
+            if (!self.has_induced_failure) {
                 mem.set(usize, &self.stack_addresses, 0);
                 var stack_trace = std.builtin.StackTrace{
                     .instruction_addresses = &self.stack_addresses,
@@ -63,6 +64,7 @@ pub const FailingAllocator = struct {
                 };
                 std.debug.captureStackTrace(return_address, &stack_trace);
                 self.has_induced_failure = true;
+            }
             return error.OutOfMemory;
         }
         const result = try self.internal_allocator.rawAlloc(len, ptr_align, len_align, return_address);