Test case helper and fix EOF detection

[lbergelson]

This started as me fixing the minor bug in #1614. I got annoyed with something that I am regularly annoyed with, needing different data providers for the severals sets of test cases I want to run even though they run the same code.

I often want to right the following test cases for the same function:

• The function matches a given value
• The function mismatches the value
• The function throws a specific exception.

I added a helper that lets you capture those cases in one data provider.

Expected.match(42)
Expected.mismatch("bleh")
Expected.exception(UserException)

It's used as an element in the data provider and then tested by calling it's test function:

  @DataProvider
    public Object[][] testCases() {
       ...                         
     {utf8File, StandardCharsets.UTF_8, Expected.match("🐋 UTF8 is Great 🐋")},
     {utf8File, StandardCharsets.ISO_8859_1, Expected.mismatch("🐋 UTF8 is Great 🐋")}
     {missingTerminator, StandardCharsets.UTF_8, Expected.exception(EOFException.class)},
     ...
   }

@Test(dataProvider = "testCases")
public void testAllCases(String filename, Charset charset, Expected<String> expected) {
     expected.test(() -> {
         try(final LittleEndianInputStream in = new LittleEndianInputStream(new BufferedInputStream(Files.newInputStream(Paths.get(filename))))){
             return in.readString(charset);
         }
     });            
} 

It seems like a useful thing to me, but I'm wanted to get feedback on the idea.

[lbergelson]

This is weirdly scoped because you're not yet allowed to put private classes in an interface.