InputTestDriver sleeps exactly until input loop emits all the test data

In order to avoid sleeping long, specify expected emits and then
InputTestDriver continuously sleeps very short so that input loop will emit all the test data.
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Can specify only the length of expected emits and assert emits after InputTestDriver#run
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Merge pull request #398 from dlackty/patch-1

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Change variable name

t0         ->
started_at
Add a comment about expected_emits_length
Merge pull request #397 from itiut/avoid-long-sleep

InputTestDriver sleeps exactly until input loop emits all the test data
Use expected_emits_length instead of sleep in in_object_space test
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Add base test for SystemConfig. ref #403
Fix TextParser to add estimate_current_event
 * to set estimate_current_event on child parser instance, between initialize and configure
   * external code cannot set it before @parser.confgure()
Merge pull request #406 from tagomoris/textparser_estimate_current_setter

Fix TextParser to add estimate_current_event
Merge pull request #404 from fluent/add-system-config-spec

Add base test for SystemConfig. ref #403
Fix bug of hash/array
 * params of hash/array can accept String only
 * params receive ruby Hash/Array objects by DSL
Fix bug for config_param of bool value without defaults
Merge pull request #410 from tagomoris/bugfix_config_checks

Bugfix of config checks
Add 'alias' option to config_section and config_param
 * To allow plugin authors to define alias name of configuration attributes
   * An valid attribute name in standard configuration syntax, MAY conflicts ruby reserved keywords in DSL
   * An valid attribute name in DSL, MAY conflicts in Fluentd configuration syntax keywords
add tests for bool value setting without default values
Merge pull request #402 from tagomoris/alias_config_attributes

Add 'alias' option to config_section and config_param