helper/resource: Add TF_ACC_LOG, TF_LOG_CORE, and TF_LOG_PROVIDER environment variable handling for Terraform versions 0.15 and later

[bflad]

Closes #992

This enables provider developers to have full control over Terraform CLI logging environment variables with the acceptance testing framework in helper/resource.

In Terraform CLI 0.15 and later, the logging environment variables work as the following:

Environment Variable	Description
TF_LOG	Global logging level for core and providers. Overrides TF_LOG_CORE (but not TF_LOG_PROVIDER) if set.
TF_LOG_CORE	Logging level for core functionality. No effect if TF_LOG is set.
TF_LOG_PROVIDER	Logging level for provider logging sent over plugin file descriptors.

To execute Terraform CLI during acceptance testing, it uses terraform-exec for all interactions. terraform-exec has certain environment variable handling rules associated with it, to prevent issues with reading stdout/stderr output from calling Terraform CLI since it needs to be able to parse that information. It was recently updated to support setting TF_LOG, TF_LOG_CORE, and TF_LOG_PROVIDER environment variables when calling Terraform, in addition to the prior support for setting TF_LOG_PATH, which defaults to always setting TF_LOG to TRACE if unset.

This enhancement allows the acceptance testing framework to accept the TF_ACC_LOG environment variable to appropriately set the TF_LOG environment variable in terraform-exec, if Terraform is version 0.15 or later. We could not change the behavior of the existing TF_LOG environment variable as it would cause undesirable side effects with the existing testing framework handling for the Go standard library log package. The TF_ACC_LOG naming follows the convention of TF_ACC_LOG_PATH setting TF_LOG_PATH. The TF_LOG value defaults to TRACE, similar to before, when TF_ACC_LOG_PATH or TF_LOG_PATH_MASK environment variables are set.

This also enables the passthrough of TF_LOG_CORE and TF_LOG_PROVIDER environment variables to terraform-exec. This is required because terraform-exec will otherwise default TF_LOG to TRACE if TF_LOG_PATH is used, meaning that without this, provider developers could not disable Terraform core logging. This enhancement opts to not prefix the variables with TF_ACC_ as there's no prior behaviors to protect and the differing environment variable names for acceptance testing is already confusing enough for everyone. They could still be prefixed for consistency with TF_ACC_LOG though, if desired.

To summarize the state of logging environment variable with the acceptance testing framework after this change:

Environment Variable	Description
TF_ACC_LOG	Controls the Terraform CLI TF_LOG environment variable
TF_ACC_LOG_PATH	Controls the Terraform CLI TF_LOG_PATH environment variable and where terraform-plugin-log SDK/provider loggers will write logs
TF_LOG	Controls the Go standard library log package log level; Any level enables terraform-plugin-log SDK/provider loggers at TRACE currently (they have to be tuned down individually at the moment)
TF_LOG_CORE	Controls the Terraform CLI TF_LOG_CORE environment variable
TF_LOG_PATH_MASK	Act similar to TF_ACC_LOG_PATH, but replaces a %s placeholder in the value with individual test names ((testing.T).Name())
TF_LOG_PROVIDER	Controls the Terraform CLI TF_LOG_PROVIDER environment variable

[bendbennett]

LGTM

[detro]

Just some typos, LGTM

[github-actions]

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