Make plan output human-readable

[radeksimko]

Some test failures may produce plan, represented as tfjson.Plan - we should to find a way to present it so it's more easily readable by humans.

--- FAIL: TestAccPacketSpotMarketRequest_Basic (439.06s)
    testing_new_config.go:52: After applying this step, the plan was not empty. (*tfjson.Plan)(0xc0000c1b60)({
         FormatVersion: (string) (len=3) "0.1",
         TerraformVersion: (string) (len=7) "0.12.20",
         Variables: (map[string]*tfjson.PlanVariable) <nil>,
         PlannedValues: (*tfjson.StateValues)(0xc0002d3840)({
          Outputs: (map[string]*tfjson.StateOutput) <nil>,
          RootModule: (*tfjson.StateModule)(0xc0004bc8c0)({
           Resources: ([]*tfjson.StateResource) (len=2 cap=4) {
            (*tfjson.StateResource)(0xc0000d4e60)({
             Address: (string) (len=19) "packet_project.test",
             Mode: (tfjson.ResourceMode) (len=7) "managed",
             Type: (string) (len=14) "packet_project",
             Name: (string) (len=4) "test",
             Index: (interface {}) <nil>,
             ProviderName: (string) (len=6) "packet",
             SchemaVersion: (uint64) 0,
             AttributeValues: (map[string]interface {}) (len=8) {
              (string) (len=16) "backend_transfer": (bool) false,
              (string) (len=10) "bgp_config": ([]interface {}) {
              },
              (string) (len=7) "created": (string) (len=20) "2020-01-29T17:26:48Z",
              (string) (len=2) "id": (string) (len=36) "96a7b2ee-c814-4b45-b607-1719757ecf49",
              (string) (len=4) "name": (string) (len=36) "tfacc-spot_market_request-epxyk7xzsv",
              (string) (len=15) "organization_id": (string) (len=36) "...",
              (string) (len=17) "payment_method_id": (string) (len=36) "...",
              (string) (len=7) "updated": (string) (len=20) "2020-01-29T17:32:54Z"
             },
...

[radeksimko]

This is actually more about terraform-json not having String() on Plan - I will look into that.

[radeksimko]

Closing in favour of hashicorp/terraform-json#14