Add ability to create a Sandbox SiddhiAppRuntime for testing purposes.

[suhothayan]

Add createSandboxSiddhiAppRuntime() to SiddhiManager to create a SiddhiApp without sources, sinks, and stores. such that it can be used for testing.

Fixes #1438

[codecov-io]

Codecov Report

Merging #1451 into master will increase coverage by 0.09%.
The diff coverage is 93.33%.

@@             Coverage Diff              @@
##             master    #1451      +/-   ##
============================================
+ Coverage     67.75%   67.85%   +0.09%     
  Complexity       41       41              
============================================
  Files           634      634              
  Lines         26792    26807      +15     
  Branches       4232     4236       +4     
============================================
+ Hits          18154    18190      +36     
+ Misses         6868     6846      -22     
- Partials       1770     1771       +1

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/io/siddhi/core/SiddhiManager.java	62.96% <93.33%> (+6.9%)	0 <0> (ø)	⬇️
.../siddhi/core/util/event/handler/StreamHandler.java	62% <0%> (-12%)	0% <0%> (ø)
...ore/util/parser/helper/DefinitionParserHelper.java	74.89% <0%> (+0.41%)	0% <0%> (ø)	⬇️
...n/java/io/siddhi/core/stream/output/sink/Sink.java	72.93% <0%> (+0.75%)	0% <0%> (ø)	⬇️
.../io/siddhi/core/util/snapshot/SnapshotService.java	72.46% <0%> (+0.82%)	0% <0%> (ø)	⬇️
...ava/io/siddhi/core/stream/input/source/Source.java	84.05% <0%> (+1.44%)	0% <0%> (ø)	⬇️
.../io/siddhi/core/util/transport/InMemoryBroker.java	88.37% <0%> (+2.32%)	0% <0%> (ø)	⬇️
...java/io/siddhi/core/stream/input/InputHandler.java	80.64% <0%> (+3.22%)	0% <0%> (ø)	⬇️
.../io/siddhi/core/stream/output/sink/SinkMapper.java	42.1% <0%> (+7.01%)	0% <0%> (ø)	⬇️
...java/io/siddhi/core/stream/input/InputManager.java	91.3% <0%> (+8.69%)	0% <0%> (ø)	⬇️
... and 4 more

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