Add simple e2e tests for all command

[austinvalle]

• Added a new test that runs off the main all command and compares a temporary test directory with the golden output directory in testdata. Moved the existing example/ir.json and used this as the first test entry.
• Skipped comparing nested directory files in the test for now 👍🏻
• Updated CLI parsing to use flagset to allow parallel tests to work

Example test failure if golden files mismatch with outputted files

Running tool: /opt/homebrew/bin/go test -timeout 30s -run ^TestAll$ github/hashicorp/terraform-provider-code-generator/internal/cmd

--- FAIL: TestAll (0.00s)
    --- FAIL: TestAll/basic (0.01s)
        /Users/austin.valle/code/terraform-plugin-codegen-framework/internal/cmd/cmd_test.go:46: unexpected difference in example_data_source_gen.go:   (
              	"""
              	... // 15 identical lines
              	var exampleDataSourceSchema = schema.Schema{
              		Attributes: map[string]schema.Attribute{
            + 			"fake_attribute": schema.BoolAttribute{
            + 				CustomType: my_boolattribute_type,
            + 				Optional:   true,
            + 			},
              			"bool_attribute": schema.BoolAttribute{
              				CustomType: my_boolattribute_type,
              	... // 76 identical lines
              	"""
              )
        /Users/austin.valle/code/terraform-plugin-codegen-framework/internal/cmd/cmd_test.go:46: unexpected difference in example_provider_gen.go:   (
              	"""
              	... // 7 identical lines
              	)
              	
            + 	// This comment doesn't belong here!
              	var exampleProviderSchema = schema.Schema{}
              	"""
              )
        /Users/austin.valle/code/terraform-plugin-codegen-framework/internal/cmd/cmd_test.go:46: unexpected difference in example_resource_gen.go:   (
              	"""
              	... // 13 identical lines
              	var exampleResourceSchema = schema.Schema{
              		Attributes: map[string]schema.Attribute{
            + 			"fake_attribute": schema.BoolAttribute{
            + 				CustomType: my_type,
            + 				Computed:   true,
            + 			},
              			"bool_attribute": schema.BoolAttribute{
              				CustomType: my_type,
              	... // 9 identical lines
              	"""
              )
FAIL
FAIL	github/hashicorp/terraform-provider-code-generator/internal/cmd	0.289s
FAIL

[bendbennett]

LGTM 🚀

[bendbennett]

Nice. TIL there's a cli.NewMockUi().

[austinvalle]

This will need to be updated after hashicorp/terraform-plugin-codegen-spec#26 is merged

[github-actions]

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