增加测试用例框架及测试用例

[BernardXiong]

围绕着PSE51相关函数，添加测试用例框架及测试用例。

目前已经在pc （ubuntu 20.04.1）上测试通过（cd testcases; ./build pc ; ./test pc）。

• ./build pc

Building PC tests...
✓ PC tests built successfully

• ./test pc

=== Running PC Tests ===

── Testing assert ──
  ✓ assert test passed

── Testing ctype ──
  ✓ ctype test passed

── Testing fcntl ──
  ✓ fcntl test passed

── Testing locale ──
  ✓ locale test passed

── Testing signal ──
  ✓ signal test passed

── Testing stat ──
  ✓ stat test passed

── Testing stdio ──

── Testing stdlib ──
  ✓ test_environment_utils
  ✓ test_integer_math
  ✓ test_memory
  ✓ test_program_control
  ✓ test_random
  ✓ test_string_conversion

── Testing string ──
  ✓ test_memory_ops
  ✓ test_string_compare
  ✓ test_string_concat
  ✓ test_string_copy
  ✓ test_string_misc
  ✓ test_string_search

── Testing time ──
  ✓ time test passed

── Testing unistd ──
  ✓ unistd test passed

── Testing utsname ──
  ✓ utsname test passed

=== PC Test Summary ===
Total: 12, Passed: 12, Failed: 0
✓ All PC tests passed!

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[unicornx]

@Chris-godz please review and comment, thanks. 另外请看一下是否可以将以前你曾经做过的 #52 的工作合并到这个 pr 里面来一起？

[BernardXiong]

Please provide your comments, thanks.