Skip un-used variable block attributes

[bendrucker]

When calling helper.TestRunner, variable blocks with any attribute other than default will trigger a parsing error. The runner needs to read the default and other attributes should silently pass through. Currently, they cause a panic.

With the modified test:

--- FAIL: Test_EvaluateExpr (0.00s)
    --- FAIL: Test_EvaluateExpr/string_interpolation (0.00s)
panic: Failed to initialize runner: main.tf:3,2-6: Unsupported argument; An argument named "type" is not expected here. [recovered]
	panic: Failed to initialize runner: main.tf:3,2-6: Unsupported argument; An argument named "type" is not expected here.

goroutine 6 [running]:
testing.tRunner.func1.2({0x12fb300, 0xc000049e20})
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1392 +0x39f
panic({0x12fb300, 0xc000049e20})
	/usr/local/Cellar/go/1.18/libexec/src/runtime/panic.go:838 +0x207
github.com/terraform-linters/tflint-plugin-sdk/helper.TestRunner(0xc0001096c0, 0xc000067e20?)
	/Users/ben/src/terraform-linters/tflint-plugin-sdk/helper/testing.go:40 +0x394
github.com/terraform-linters/tflint-plugin-sdk/helper.Test_EvaluateExpr.func1(0xc0001096c0)
	/Users/ben/src/terraform-linters/tflint-plugin-sdk/helper/runner_test.go:281 +0x105
testing.tRunner(0xc0001096c0, 0xc000049d50)
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/local/Cellar/go/1.18/libexec/src/testing/testing.go:1486 +0x35f
FAIL	github.com/terraform-linters/tflint-plugin-sdk/helper	0.137s
FAIL

Using PartialContent rather than Content resolves the failed test.

Closes #152