Fix broken flattenIap method for backend service and improve import test

[rosbo]

Continuing the work started in #881.

While adding more import test, I caught a bug. The flattenIAP method was broken and was setting a field that do not exists causing the d.Set call to fail.

Delete dedicated import tests
Add more import test (++coverage)
Make it faster to run all tests

[rosbo]

--- PASS: TestAccComputeBackendService_withCDNEnabled (44.04s)
--- PASS: TestAccComputeBackendService_withConnectionDraining (44.93s)
--- PASS: TestAccComputeBackendService_withHttpsHealthCheck (44.94s)
--- PASS: TestAccComputeBackendService_updatePreservesOptionalParameters (55.61s)
--- PASS: TestAccComputeBackendService_withSessionAffinity (55.74s)
--- PASS: TestAccComputeBackendService_withConnectionDrainingAndUpdate (55.96s)
--- PASS: TestAccComputeBackendService_basic (66.63s)
--- PASS: TestAccComputeBackendService_withBackend (156.04s)
--- PASS: TestAccComputeBackendService_withBackendAndIAP (155.37s)

[ghost]

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