Preserve quoted trailing whitespace in config values

[ctoth]

Follow-up to #2144.

The initial Hypothesis config round-trip property exposed a case where quoted trailing whitespace was not preserved when reading config values back.

For example, a value like b" " is written as a quoted config value, but parsing discarded the whitespace because _parse_string buffered whitespace even while inside quotes.

This changes _parse_string so spaces and tabs inside quotes are preserved immediately, while trailing whitespace outside quotes is still ignored.

Also included:

• a focused regression test for round-tripping quoted trailing space and tab values
• removal of the temporary property-test filter that excluded values ending in space or tab

[jelmer]

Nice, thanks!