Add unittest

Add unittest

[   34s] + ./cexpr
[   34s]
[   34s] running 6 tests
[   34s] test int_signed ... FAILED
[   34s] test chars ... FAILED
[   34s] test fail ... FAILED
[   34s] test floats ... FAILED
[   34s] test int_unsigned ... FAILED
[   34s] test strings ... FAILED
[   34s]
[   34s] failures:
[   34s]
[   34s] ---- int_signed stdout ----
[   34s] thread 'int_signed' panicked at 'Once instance has previously been poisoned', ./tests/clang.rs:317:15
[   34s]
[   34s] ---- chars stdout ----
[   34s] thread 'chars' panicked at 'a `libclang` shared library is not loaded on this thread', ./src/lib.rs:1735:1
[   34s] note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[   34s]
[   34s] ---- fail stdout ----
[   34s] thread 'fail' panicked at 'Once instance has previously been poisoned', ./tests/clang.rs:317:15
[   34s]
[   34s] ---- floats stdout ----
[   34s] thread 'floats' panicked at 'Once instance has previously been poisoned', ./tests/clang.rs:317:15
[   34s]
[   34s] ---- int_unsigned stdout ----
[   34s] thread 'int_unsigned' panicked at 'Once instance has previously been poisoned', ./tests/clang.rs:317:15
[   34s]
[   34s] ---- strings stdout ----
[   34s] thread 'strings' panicked at 'Once instance has previously been poisoned', ./tests/clang.rs:317:15
[   34s]
[   34s]
[   34s] failures:
[   34s]     chars
[   34s]     fail
[   34s]     floats
[   34s]     int_signed
[   34s]     int_unsigned
[   34s]     strings
[   34s]
[   34s] test result: FAILED. 0 passed; 6 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s