docs: Add documentation about tests and SBIUnit

This patch contains the documentation for SBIUnit. It describes:

- What is SBIUnit
- Simple test writing scenario
- How we can cover static functions
- How we can "mock" structures in order to test the functions which
operate on them
- SBIUnit API Reference

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

docs/doxygen.cfg

@@ -797,6 +797,7 @@ INPUT                  = @@SRC_DIR@@/README.md \
                          @@SRC_DIR@@/docs/library_usage.md \
                          @@SRC_DIR@@/docs/domain_support.md \
                          @@SRC_DIR@@/docs/opensbi_config.md \
+                         @@SRC_DIR@@/docs/writing_tests.md \
                          @@SRC_DIR@@/docs/firmware \
                          @@SRC_DIR@@/docs/platform \
                          @@SRC_DIR@@/include \