releases: add darwin/arm64 mock build

Re-Built as per instructions under releases/testdata/README.md
The older builds had to be rebuilt because of checksums & signature which are all contained in one file
and the checksums have changed for other builds because of newer version of Go.

releases/testdata/mock_api_tf_0_14_with_prereleases/terraform/index.json

@@ -612,6 +612,14 @@
                     "filename": "terraform_0.15.0-rc2_darwin_amd64.zip",
                     "url": "https://releases.hashicorp.com/terraform/0.15.0-rc2/terraform_0.15.0-rc2_darwin_amd64.zip"
                 },
+                {
+                    "name": "terraform",
+                    "version": "0.15.0-rc2",
+                    "os": "darwin",
+                    "arch": "arm64",
+                    "filename": "terraform_0.15.0-rc2_darwin_arm64.zip",
+                    "url": "https://releases.hashicorp.com/terraform/0.15.0-rc2/terraform_0.15.0-rc2_darwin_arm64.zip"
+                },
                 {
                     "name": "terraform",
                     "version": "0.15.0-rc2",

releases/testdata/mock_terraform_builds/0.15.0-rc2/terraform_0.15.0-rc2_SHA256SUMS

@@ -1,3 +1,4 @@
-518baaa2b8926bf704d5f72f50631b46f598fb883da6691dd5523091ed54bae3  terraform_0.15.0-rc2_darwin_amd64.zip
-d4ede4731c2d5f30d786cb9a28004d77037b0bc25e3343e82536509b4dd8d13c  terraform_0.15.0-rc2_linux_amd64.zip
-eb6c292c28058fd0aea75ef1a14cda16fa817c3e5bd0df08cd74d565eb556bcf  terraform_0.15.0-rc2_windows_amd64.zip
+d432b9722de43e89bb1568344b47c707348025d809765817ece2e46cd0d7b24d  terraform_0.15.0-rc2_darwin_amd64.zip
+6b6bf2497aefbef79076bde41a0718b72e9adabb3e33fafc45e9494546199e43  terraform_0.15.0-rc2_darwin_arm64.zip
+44dfce5c083a9187393262a5071e21e55d1c38dc4bd2a6c5284ae7924974be8a  terraform_0.15.0-rc2_linux_amd64.zip
+14a5acf8b7dd8c6947e6af633e7962f0d4974c4ac0eb0fcefb1ff43db829ea1f  terraform_0.15.0-rc2_windows_amd64.zip