Don't crash when dependent test sweeper is missing

[ewbankkit]

Closes #278.

Without the fix the new test cases cause a crash:

$ go test ./helper/resource/...
2019/12/12 07:57:21 [DEBUG] New state was assigned lineage "7d80f5ca-2c51-4290-c29e-0df3b7ff4e30"
2019/12/12 07:57:21 [DEBUG] Waiting for state to become: [done]
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [DEBUG] Waiting for state to become: [done]
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:21 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [WARN] WaitForState timeout after 85ms
2019/12/12 07:57:22 [WARN] WaitForState starting 30s refresh grace period
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: [running]
2019/12/12 07:57:22 [WARN] WaitForState timeout after 1ms
2019/12/12 07:57:22 [WARN] WaitForState starting 5ms refresh grace period
2019/12/12 07:57:22 [ERROR] WaitForState exceeded refresh grace period
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: [running]
2019/12/12 07:57:22 [WARN] WaitForState timeout after 10ms
2019/12/12 07:57:22 [WARN] WaitForState starting 30s refresh grace period
2019/12/12 07:57:22 [TRACE] Waiting 10s before next try
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: [running]
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: [done]
2019/12/12 07:57:22 [TRACE] Waiting 200ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 400ms before next try
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: []
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: []
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [TRACE] Waiting 10ms before next try
2019/12/12 07:57:22 [WARN] WaitForState timeout after 100ms
2019/12/12 07:57:22 [WARN] WaitForState starting 30s refresh grace period
2019/12/12 07:57:22 [DEBUG] Waiting for state to become: [running]
	- aws_dummy
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xf762dc]

goroutine 94 [running]:
testing.tRunner.func1(0xc000475e00)
	/usr/local/go/src/testing/testing.go:874 +0x3a3
panic(0x10ed6a0, 0x1d8a6c0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
github.com/hashicorp/terraform-plugin-sdk/helper/resource.filterSweeperWithDependencies(0x127c99a, 0x10, 0xc000486de0, 0x10)
	/home/kit/wrk/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/testing.go:199 +0xcc
github.com/hashicorp/terraform-plugin-sdk/helper/resource.filterSweeperWithDependencies(0x127b6b1, 0xf, 0xc000486de0, 0x0)
	/home/kit/wrk/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/testing.go:200 +0x1b0
github.com/hashicorp/terraform-plugin-sdk/helper/resource.TestFilterSweeperWithDependencies.func1(0xc000475e00)
	/home/kit/wrk/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/testing_test.go:1243 +0x5d
testing.tRunner(0xc000475e00, 0xc000496640)
	/usr/local/go/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:960 +0x350
FAIL	github.com/hashicorp/terraform-plugin-sdk/helper/resource	0.907s
FAIL

[radeksimko]

Hi @ewbankkit
Good catch!

I'm happy with the implementation functionally, I just left you one comment inline to address.

[radeksimko]

LGTM, thanks!

[radeksimko]

@kmoe - I reckon this doesn't touch the part of test framework you're currently working on, but FYI

[ghost]

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