Simplify tests by figuring out fixture dir automatically

We always use the same directory for terraform fixtures, so it's quite
easy to figure out its path from a pytest fixture by inspecting the
stack. This commit implements this functionality and decreases the
amount of boilerplate needed to write a test.

(Ported from fast)

.gitignore

@@ -17,4 +17,5 @@ terraform-ls.tf
 bundle.zip
 .DS_Store
 **/packer_cache
-**/*.pkrvars.hcl
+**/*.pkrvars.hcl
+fixture_*