BaseTools/Plugin: Add coverage support for Unit Test

BaseTools/Plugin: Add coverage support for Unit Test

For GCC, use lcov to generate Unit Test code coverage
report

For VS2019, use OpenCppCoverage to generate code
coverage report

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <gaoliming@bysoft.com.cn>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael Kubacki <mikuback@linux.microsoft.com>
Signed-off-by: Gua Guo <gua.guo@intel.com>