libc: add memset test for int-to-char conversion

[strajabot]

Summary

Test case to check if an implementation of memset correctly
handles the value passed being wider than a byte

[strajabot]

Larger buffer size better tests how the parameter is handled

[fuz]

Test looks reasonable. Were you able to trigger test suite failure with your broken memset?

lib/libc/tests/string/memset_test.c

6–26

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