Improve line and branch coverage for Http policy related files.

[ahsonkhan]

Our line coverage bar for the repo is is 90% and branch coverage is 70%.

Though HTTP pipeline and policies are challenging to add unit tests for (and in some cases require live tests that run nightly to improve coverage), these files are significantly below our current bar:
https://dev.azure.com/azure-sdk/internal/_build/results?buildId=2104943&view=codecoverage-tab

For example, the az_http_pipeline_policy_credential() function isn't covered by any test.

azure-sdk-for-c/sdk/src/azure/core/az_http_policy.c

Lines 97 to 113 in c3bfd14

	AZ_NODISCARD az_result az_http_pipeline_policy_credential(
	_az_http_policy* ref_policies,
	void* ref_options,
	az_http_request* ref_request,
	az_http_response* ref_response)
	{
	_az_credential* const credential = (_az_credential*)ref_options;
	_az_http_policy_process_fn const policy_credential_apply
	= credential == NULL ? NULL : credential->_internal.apply_credential_policy;
	if (credential == AZ_CREDENTIAL_ANONYMOUS || policy_credential_apply == NULL)
	{
	return _az_http_pipeline_nextpolicy(ref_policies, ref_request, ref_response);
	}
	return policy_credential_apply(ref_policies, credential, ref_request, ref_response);
	}

This becomes necessary to support services that use the HTTP protocol such as storage, which we currently have as part of a feature branch.

azure-sdk-for-c/sdk/src/azure/storage/az_storage_blobs_blob_client.c

Lines 147 to 157 in 5c6fa6e

	{
	._internal = {
	.process = az_http_pipeline_policy_retry,
	.options = &out_client->_internal.options.retry_options,
	},
	},
	{
	._internal = {
	.process = az_http_pipeline_policy_credential,
	.options = cred,
	},

Brought up in #2443 (comment)

cc @antkmsft

[CIPop]

Hidden by MicrosoftPremier/VstsExtensions#219

Blocked #2591 : we're working around by temporarily lowering the thresholds in feature/v2 until each file is at required levels.