tests: shell_flash: fix test case scenario

The previous test had two defects. First, the shell command was called
before the backend was initialized.

Second, the test compared the string from shell's backend buffer with
the reference string using strncmp function. This test result was very
susceptible to the time when it was performed because of data contained
in the buffer. Apart from expected string there was data like escape
codes needed to control the terminal.

To fix it, function strncmp was replaced by strstr, which
searches for the reference strings regardless of the characters
controlling the terminal.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordisemi.no>