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/filesystem.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"io/ioutil"
+	"os"
+)
+
+func main() {
+	_, err := os.Open("non-exist")
+	if !os.IsNotExist(err) {
+		panic("should be non exist error")
+	}
+
+	f, err := os.Open("testdata/filesystem.txt")
+	if err != nil {
+		panic(err)
+	}
+
+	defer func() {
+		if err := f.Close(); err != nil {
+			panic(err)
+		}
+
+		// read after close: error should be returned
+		_, err := f.Read(make([]byte, 10))
+		if err == nil {
+			panic("error expected for reading after closing files")
+		}
+	}()
+
+	data, err := ioutil.ReadAll(f)
+	if err != nil {
+		panic(err)
+	}
+
+	print(string(data))
+
+}