oeqa/selftest/rust: Simplify the rust testsuite output gathering/processing

oeqa/selftest/rust: Simplify the rust testsuite output gathering/processing

The rust testsuite was redirecting command output to a file, which made it
hard to debug failure cases since the logs were not available to print to
the console.

Rework the code so it uses the existing popen logging and hence allows us
to improve the error logging situation and make debugging failures easier.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>