Use NOBENCHMARK env

[aycabta]

This Pull Request is modified versions of https://github.com/rdoc/rdoc/pull/446 what was not refined.

The command rake runs tests with benchmark:

$ rake
Run options: --seed 35482

# Running tests:

......(snip)......

Finished tests in 13.490933s, 159.9593 tests/s, 381.0707 assertions/s.

2158 tests, 5141 assertions, 0 failures, 0 errors, 1 skips

# Running benchmarks:


TestRDocContext 1       10      100     1000    10000
bench_add_include        0.000037        0.000052        0.000288        0.003270        0.020100


Finished benchmarks in 0.084059s, 11.8964 tests/s, 11.8964 assertions/s.

1 tests, 1 assertions, 0 failures, 0 errors, 1 skips

The NOBENCHMARK=1 rake runs tests without benchmark:

$ NOBENCHMARK=1 rake
Run options: --seed 274

# Running tests:

......(snip)......

Finished tests in 21.829784s, 98.8558 tests/s, 235.5039 assertions/s.

2158 tests, 5141 assertions, 0 failures, 0 errors, 1 skips

This is easily understandable.

[zzak]

@aycabta Thanks! Could you squash the commits please?

[aycabta]

@zzak OK, done.