Split tests into separate files, add secret tests

tests/01_environment.bats

@@ -0,0 +1,17 @@
+#!/usr/bin/env bats
+
+@test "docker is available" {
+    command -v docker
+}
+
+@test "abra is executable" {
+    ./abra --help
+}
+
+@test "git is available" {
+    command -v git
+}
+
+@test "running in a container" {
+    ls /etc/alpine-release
+}