dts: dtlib: Fix parsing of character literals

This was allowed due to a misunderstanding:

    foo = 'x';

In reality, 'x' works like an integer literal, and is used like this:

    foo = < 'x' >;

Fix character literal parsing to match the C tools.

Also fix backslash escape parsing to match the C tools exactly
(get_escape_char() in util.c): \<char> should be turned into <char> if
<char> isn't recognized as a special escape character, instead of being
left alone. This fixes parsing of e.g. '\'' (a character literal with a
single quote in it).

Piggyback some more tests for weird property/node names.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>