Enable race detector in tests, fix data races

[vearutop]

This PR:

• enables -race in tests,
• adds sync accessors to test helpers fields,
• deprecates direct access to those fields.

[github-actions]

Lines Of Code

Language	Files	Lines	Code	Comments	Blanks	Complexity	Bytes
Go	28	2827 (+45)	2083 (+30)	265 (+4)	479 (+11)	379 (+4)	73.7K (+917B)
Go (test)	6	1334	1077	41	216	81	38.1K (-197B)
YAML	4	364	310	16	38	0	13.3K (+6B)

[github-actions]

Go API Changes

# github.com/adjust/rmq/v5
## compatible changes
(*TestBatchConsumer).Consumed: added
(*TestBatchConsumer).Last: added
(*TestConsumer).Deliveries: added
(*TestConsumer).Last: added

# summary
Inferred base version: v5.0.2
Suggested version: v5.1.0

[github-actions]

Benchmark Result

Benchmark diff with base branch

name     old time/op    new time/op    delta
Queue-2     176µs ± 2%     174µs ± 6%   ~     (p=0.792 n=5+6)

name     old alloc/op   new alloc/op   delta
Queue-2    45.5kB ± 3%    45.0kB ± 7%   ~     (p=0.485 n=6+6)

name     old allocs/op  new allocs/op  delta
Queue-2      87.7 ± 1%      88.0 ± 0%   ~     (p=0.545 n=6+5)

Benchmark result

name     time/op
Queue-2   174µs ± 6%

name     alloc/op
Queue-2  45.0kB ± 7%

name     allocs/op
Queue-2    88.0 ± 0%

[github-actions]

Unit Test Coverage

total: (statements) 70.1%
changed lines: (statements) 100.0%

Coverage of changed lines

File	Function	Coverage
Total		100.0%
test_batch_consumer.go		100.0%
test_batch_consumer.go:22	Last	100.0%
test_batch_consumer.go:29	Consumed	100.0%
test_batch_consumer.go:36	Consume	100.0%
test_batch_consumer.go:50	Finish	100.0%
test_consumer.go		100.0%
test_consumer.go:36	Last	100.0%
test_consumer.go:43	Deliveries	100.0%
test_consumer.go:50	Consume	100.0%

Coverage diff with base branch

File	Function	Base Coverage	Current Coverage
Total		69.4%	70.1% (+0.7%)
github.com/adjust/rmq/v5/test_batch_consumer.go	Consume	80.0%	85.7% (+5.7%)
github.com/adjust/rmq/v5/test_batch_consumer.go	Consumed	no function	100.0%
github.com/adjust/rmq/v5/test_batch_consumer.go	Last	no function	100.0%
github.com/adjust/rmq/v5/test_consumer.go	Consume	77.8%	81.8% (+4.0%)
github.com/adjust/rmq/v5/test_consumer.go	Deliveries	no function	100.0%
github.com/adjust/rmq/v5/test_consumer.go	Last	no function	100.0%