handle non-utf8 from libfuzzer stderr

[bmc-msft]

Moves to using String::from_utf8_lossy to cleanly ignore non-utf8 output from libfuzzers.

[bmc-msft]

Testing of the basic feature was done by adding the following to LLVMFuzzerTestOneInput to a sample libfuzzer target.

fprintf(stdout, "\xFF\xFF\xFF\xFF\n");
fprintf(stderr, "\xFF\xFF\xFF\xFF\n");

Then run via:

onefuzz-agent debug libfuzzer-fuzz ~/tests/libfuzzer-stdout-binary/fuzz.exe /tmp/seeds /tmp/crashes
onefuzz-agent debug libfuzzer-crash-report  ~/tests/libfuzzer-stdout-binary/fuzz.exe /tmp/crashes/crash-8462399f9f420af69de1a1862b9af328ef35971c

[bmc-msft]

This passed integration testing.