test: Use printf "%b\n" instead of echo for strings with '\'

Use of echo with special characters is not portable. On a normal
POSIX system, the behavior with backslash escaped strings is
implementation-defined. On an XSI-conformant system, they must
be interpreted.

A way out is the printf command - printf "%b" specifies that
the following argument is to be printed with backslash escapes
interpreted.

Gbp-Dch: ignore

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