testing/traefik-2.4.3: 2 test failures with go 1.16

I've reported it upstream: https://github.com/traefik/traefik/issues/7907

On most arches, traefik-2.4.3 was built with go 1.15, but because aarch64 was lagging, it was built with go 1.16 which was pushed in the mean time.

--- FAIL: TestRetry (0.00s)
    --- FAIL: TestRetry/max_attempts_exhausted_delivers_the_5xx_response_with_backoff (0.00s)
        retry_test.go:148: 
            	Error Trace:	retry_test.go:148
            	Error:      	Not equal: 
            	            	expected: 502
            	            	actual  : 200
            	Test:       	TestRetry/max_attempts_exhausted_delivers_the_5xx_response_with_backoff
    --- FAIL: TestRetry/max_attempts_exhausted_delivers_the_5xx_response (0.00s)
        retry_test.go:148: 
            	Error Trace:	retry_test.go:148
            	Error:      	Not equal: 
            	            	expected: 502
            	            	actual  : 200
            	Test:       	TestRetry/max_attempts_exhausted_delivers_the_5xx_response
FAIL
FAIL	github.com/traefik/traefik/v2/pkg/middlewares/retry	0.051s