Fix regression by allowing providers defined at TestCase level with providers defined within TestStep.Config

[bendbennett]

Closes: #176

Prior to the changes in this PR, the following test failed:

		"config-providers-testcase-providers": {
			testStep: TestStep{
				Config: "provider abc {",
			},
			testStepConfig: "# not empty",
			testStepValidateRequest: testStepValidateRequest{
				TestCaseHasProviders: true,
			},
		},

=== NAME  TestTestStepValidate/config-providers-testcase-providers
    teststep_validate_test.go:523: unexpected error: Providers must only be specified either at the TestCase or TestStep level

Whereas the equivalent test in v1.4.0 passes:

		"config-providers-testcase-providers": {
			testStep: TestStep{
				Config: "provider abc {",
			},
			testStepValidateRequest: testStepValidateRequest{
				TestCaseHasProviders: true,
			},
		},

[austinvalle]

🚀

[bflad]

Looks good to me 🚀

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