main: clean up tests

- Explicitly list all test cases. This makes it possible to store
    tests in testdata/ that aren't tested on all platforms.
  - Clean up filesystem and env test, by running them in a subtest and
    deduplicating some code and removing the additionalArgs parameter.

testdata/env.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"os"
+)
+
+func main() {
+	println(os.Getenv("ENV1"))
+	v, ok := os.LookupEnv("ENV2")
+	if !ok {
+		panic("ENV2 not found")
+	}
+	println(v)
+}