Make the tests complete earlier.

[newacorn]

Reduce the test duration to 36% of the original time.
Before Time Spend See Issue: #1839
After Commit:

~/GolandProjects/cbrotli_bug/fasthttp fasthttp_test_issue2*
❯ go test .
ok  	github.com/valyala/fasthttp	2.558s

~/GolandProjects/cbrotli_bug/fasthttp fasthttp_test_issue2*
❯ go test -count=2
PASS
ok  	github.com/valyala/fasthttp	4.976s

~/GolandProjects/cbrotli_bug/fasthttp fasthttp_test_issue2* 5s
❯ go test -count=3 .
ok  	github.com/valyala/fasthttp	7.389s

~/GolandProjects/cbrotli_bug/fasthttp fasthttp_test_issue2* 8s
❯ go test -count=4 .
ok  	github.com/valyala/fasthttp	9.991s

[erikdubbelboer]

Thanks!