Add `run-tests.php --context [n]` option.

Mentioned in https://github.com/php/php-src/pull/5965#discussion_r467621123

This PR proposes 3 lines of context so the impact can be seen in tests.
Other `diff` programs show around 3 lines of context.
(This helps indicate exactly which position a test should be updated
to add a new expected line at)

Use the mapping for choosing order to display diffs

Properly include context in cases where the expected output had more lines than
the actual output, e.g.

```
--FILE--
A
A1
A
C
NEARBY
--EXPECTF--
A
B
A1
B
A
B
A
B
NEARBY
```

Closes GH-5968

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