Create FileHandle mock tests to enable testing of cuFile I/O path on non-GDS systems

[kingcrimsontianyu]

This PR is WIP

This PR by dint of the gmock framework enables the testing of different compatibility modes uniformly on a non-GDS system for the core class FileHandle. The following cases have been mocked:

• FileHandle constructor receives argument CompatMode::ON.
• FileHandle constructor receives argument CompatMode::OFF. The file cannot be opened with O_DIRECT flag, and an exception is thrown.
• FileHandle constructor receives argument CompatMode::AUTO. The file cannot be opened with O_DIRECT flag, and the compatibility mode is set to ON.

In addition, this PR slightly improves existing tests by:

• Removing the hand-written main() and instead linking to the gtest/gmock-provided main according to gtest's recommendation;
• Regrouping tests into different binaries, namely BASIC_IO_TEST, DEFAULTS_TEST, FILE_HANDLE_TEST;
• Fixing the file permission definition 644 in FileHandle by removing S_IWGRP.

Addresses #556

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