terraform: fix ContextPlan test failure on Windows
terraform: fix ContextPlan test failure on Windows

The attributes in the diff are %#v-formatted. This means that all `\`
characters in the Windows paths are escaped with a `\`. We need to
escape the `\` characters in cwd, module, and root before doing any
comparison work.

Fixes the following test failure on Windows:

--- FAIL: TestContextPlan_pathVar (0.00s)
        context_test.go:3833: bad:
                DIFF:

                CREATE: aws_instance.foo
                  cwd:    "" => "C:\\Users\\ceh\\src\\github.com\\hashicorp\\terraform\\terraform/barpath"
                  module: "" => "C:\\Users\\ceh\\src\\github.com\\hashicorp\\terraform\\terraform\\test-fixtures\\plan-path-var/foopath"
                  root:   "" => "C:\\Users\\ceh\\src\\github.com\\hashicorp\\terraform\\terraform\\test-fixtures\\plan-path-var/barpath"
                  type:   "" => "aws_instance"

                STATE:

                <no state>

                expected:

                DIFF:

                CREATE: aws_instance.foo
                  cwd:    "" => "C:\Users\ceh\src\github.com\hashicorp\terraform\terraform/barpath"
                  module: "" => "C:\Users\ceh\src\github.com\hashicorp\terraform\terraform\test-fixtures\plan-path-var/foopath"
                  root:   "" => "C:\Users\ceh\src\github.com\hashicorp\terraform\terraform\test-fixtures\plan-path-var/barpath"
                  type:   "" => "aws_instance"

                STATE:

                <no state>
FAIL
exit status 1
FAIL    github.com/hashicorp/terraform/terraform        0.050s

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk