If the environment variable MBEDTLS_TEST_OUTCOME_FILE is set, then for
each test case, write a line to the file with the given name, of the
form
PLATFORM;CONFIGURATION;TEST SUITE;TEST CASE DESCRIPTION;PASS/FAIL/SKIP;CAUSE
PLATFORM and CONFIGURATION come from the environment variables
MBEDTLS_TEST_PLATFORM and MBEDTLS_TEST_CONFIGURATION.
Errors while writing the test outcome file are not considered fatal,
and are not reported except for an error initially opening the file.
This is in line with other write errors that are not checked.
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