feat: add tests for signed char

Signed-off-by: krishna2803 <[email protected]>

Author: krishna2803

libc/test/UnitTest/LibcTest.cpp

@@ -223,6 +223,7 @@ TEST_SPECIALIZATION(int);
 TEST_SPECIALIZATION(long);
 TEST_SPECIALIZATION(long long);
 
+TEST_SPECIALIZATION(signed char);
 TEST_SPECIALIZATION(unsigned char);
 TEST_SPECIALIZATION(unsigned short);
 TEST_SPECIALIZATION(unsigned int);