Fix flaky TestIngester_Push_CircuitBreaker_DeadlineExceeded test

[pr00se]

This test would sometimes fail:

--- FAIL: TestIngester_Push_CircuitBreaker_DeadlineExceeded (2.00s)
    --- FAIL: TestIngester_Push_CircuitBreaker_DeadlineExceeded/test_slow_push_with_initial_delay_disabled (2.00s)
        circuitbreaker_test.go:1015: 
            	Error Trace:	/go/src/github.com/grafana/mimir/pkg/ingester/circuitbreaker_test.go:1015
            	Error:      	Received unexpected error:
            	           	circuit breaker open on push request type with remaining delay 9.999979667s
            	Test:       	TestIngester_Push_CircuitBreaker_DeadlineExceeded/test_slow_push_with_initial_delay_disabled

The test assumes that it will execute one successful request, and then 4 failing requests. However, sometimes the first request would take longer to execute than the configured 100ms timeout, and the circuit breaker would close after the second overall request, rather than after the third.

The trivial fix is to increase the push timeout to a larger value, so that the initial request has time to complete successfully, even if the test runner is slow. This PR bumps it to 1s.

With this change I was able to run this test 1000x without failure locally:

go test -timeout 300m ./pkg/ingester/... -run "TestIngester_Push_CircuitBreaker_DeadlineExceeded" -race -count 1000 -failfast
ok  	github.com/grafana/mimir/pkg/ingester	4026.572s
ok  	github.com/grafana/mimir/pkg/ingester/activeseries	1.018s [no tests to run]
ok  	github.com/grafana/mimir/pkg/ingester/activeseries/model	1.018s [no tests to run]
ok  	github.com/grafana/mimir/pkg/ingester/client	1.047s [no tests to run]
make: Leaving directory '/go/src/github.com/grafana/mimir'

real	67m16.581s
user	0m0.121s
sys	0m0.026s

[duricanikolic]

LGTM
Thank you