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fix(internal/syncwriter): write diagnostics in a single os.Stderr write (#226)

The error handler used the builtin println, which writes to fd 2
unsynchronized with the test stream and emits the message and its
trailing newline as two separate syscalls. Under concurrency a test
framework's "--- PASS:"/"--- FAIL:" line can land between those two
writes, so test2json never records that test's terminal action and
tools like gotestsum report an innocent, passing test as unknown or
failed.

Format the message and its newline into one fmt.Fprintf to os.Stderr so
the write can't be split, which keeps the result marker at the start of
a line.

Fixes #225.

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896 of 933 relevant lines covered (96.03%)

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