Re-write the RetryPolicy unit tests to remove strong coupling with implementation detail which are marked as virtual for mocking

[ahsonkhan]

Follow-up from #5584
Currently the RetryPolicy has these two methods which are part of its implementation detail and being used in unit tests:

• ShouldRetryOnResponse
• ShouldRetryOnTransportFailure

azure-sdk-for-cpp/sdk/core/azure-core/inc/azure/core/http/policies/policy.hpp

Lines 405 to 417 in ac68505

	protected:
	virtual bool ShouldRetryOnTransportFailure(
	RetryOptions const& retryOptions,
	int32_t attempt,
	std::chrono::milliseconds& retryAfter,
	double jitterFactor = -1) const;
	virtual bool ShouldRetryOnResponse(
	RawResponse const& response,
	RetryOptions const& retryOptions,
	int32_t attempt,
	std::chrono::milliseconds& retryAfter,
	double jitterFactor = -1) const;

These methods don't need to be virtual nor are they part of the public surface area. However, the way the tests are written, they take a dependency on this implementation detail. Additionally, because the unit tests have them mocked, they have to be marked as virtual, which they shouldn't be.

azure-sdk-for-cpp/sdk/core/azure-core/test/ut/retry_policy_test.cpp

Lines 107 to 126 in ac68505

	protected:
	bool ShouldRetryOnTransportFailure(
	RetryOptions const& retryOptions,
	int32_t attempt,
	std::chrono::milliseconds& retryAfter,
	double jitterFactor) const override
	{
	return m_shouldRetryOnTransportFailure(retryOptions, attempt, retryAfter, jitterFactor);
	}
	bool ShouldRetryOnResponse(
	RawResponse const& response,
	RetryOptions const& retryOptions,
	int32_t attempt,
	std::chrono::milliseconds& retryAfter,
	double jitterFactor) const override
	{
	return m_shouldRetryOnResponse(response, retryOptions, attempt, retryAfter, jitterFactor);
	}
	};