sync: improve output for choosing tests

[ee7]

Summary:

• Move the user's response to the end of the line. It was previously on
  a new line.
• Remove some unmatched double quotes, and some strange colons.
• Prompt on stderr.
• Add two blank link between test cases.

Before this commit:

[warn] isogram: missing 2 test cases
The following test case is missing:"
{
  (omitted for clarity)
}:

Do you want to include the test case ([y]es/[n]o/[s]kip)?:

s
[warn] kindergarten-garden: missing 3 test cases

With this commit:

[warn] isogram: missing 2 test cases
The following test case is missing:
{
  (omitted for clarity)
}

Do you want to include the test case ([y]es/[n]o/[s]kip)? s


[warn] kindergarten-garden: missing 3 test cases

---

In the future, this is an area where I'd like to add color for clarity (potentially with syntax highlighting for JSON) - I've opened #353.

[ErikSchierboom]

I really like it!