encoding/toml: mimic indented CUE multi-line strings in test strings

This way, when we start reporting positions in errors,
we use line and column numbers that visually make sense for the reader,
rather than always beginning from the second line and indentation.

Note that we indent the closing quote by one more level,
to match CUE's indentation semantics for multiline strings.

No change in the test results here; this change is separated to make
reviewing the following patches easier.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I1bf6f9422b623c4c6458290b448efde9330b7d88
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199024
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>