Without this patch there are no spec tests for the sensu_check JSON provider.
This is problem because a reference is needed to specify the expected behavior
of all providers.

This patch implements a pattern of stubbing out the filesystem.  All reads and
writes in the provider itself are routed through the `read_file` and
`write_json_object` methods.  The RSpec tests then use rspec-mocks to stub out
the reads and set expectations on the output.

This reference may be applied to any provider using the `flush` method.  The
setter methods in the provider for each property are expected to update state in
an instance variable, conventionally named @property_flush but named @conf in
the sensu_check provider.  The flush method is responsible for writing out
@property_flush (@conf), which we intercept and set expectations on the data
provided.

N.B. There is a bug with Rspec where the expected and actual values of
multi-line strings will not have a nice diff output if the two strings disagree
on the presence of the trailing newline.  See
rspec/rspec-support#70 for more information.  Because
of this issue in combination with the use of IO#puts in the write_output class
method, care should be taken with the examples to make sure the expected and
actual values agree on the trailing newline.  In this patch, the fixture data
for the expected output is chomp()'ed to match the string passed to
write_output().

Resolves #759

(GH-578) etc_dir should be configurable

(#759) Add reference spec tests for sensu_check JSON provider

WIP 582  Don't do type convertion on keys of sensu_client_config custom param

Add handle_silenced parameter to handler defined type

Validates that these files are JSON files.

Add validation of spec/fixtures/unit/**/*.json