Tests fail on CI due to Terraform not being installed

[dansimau]

These fixtures need a terraform: block so that astro downloads Terraform.

We should probably also cache the .tvm directory.

--- FAIL: TestNoVariables (0.00s)
	Error Trace:	flags_test.go:29
	Error:      	Received unexpected error:
	            	failed to load YAML from file: fixtures/flags/no_variables.yaml; exec: "terraform": executable file not found in $PATH
	Test:       	TestNoVariables
--- FAIL: TestSimpleVariables (0.00s)
	Error Trace:	flags_test.go:39
	Error:      	Received unexpected error:
	            	failed to load YAML from file: fixtures/flags/simple_variables.yaml; exec: "terraform": executable file not found in $PATH
	Test:       	TestSimpleVariables
--- FAIL: TestMergeValues (0.00s)
	Error Trace:	flags_test.go:70
	Error:      	Received unexpected error:
	            	failed to load YAML from file: fixtures/flags/merge_values.yaml; exec: "terraform": executable file not found in $PATH
	Test:       	TestMergeValues