[clang-tools-extra] Document clang tidy unit tests target

[Eugene.Zelenko]

clang-tools-extra/docs/clang-tidy/Contributing.rst

367

Clang-tidy

[aaron.ballman]

LGTM!

clang-tools-extra/docs/clang-tidy/Contributing.rst

369

[LegalizeAdulthood]

Update from review comments

clang-tools-extra/docs/clang-tidy/Contributing.rst

 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 Private custom matchers are a good example of auxiliary support code for your check
 that can be tested with a unit test.  It will be easier to test your matchers or
 other support classes by writing a unit test than by writing a ``FileCheck`` integration
 test.  The ``ASTMatchersTests`` target contains unit tests for the public AST matcher
 classes and is a good source of testing idioms for matchers.
 
+You can build the Clang-tidy unit tests by building the ``ClangTidyTests`` target.
+Test targets in LLVM and Clang are excluded from the "build all" style action of
+IDE-based CMake generators, so you need to explicitly build the target for the unit
+tests to be built.
+
 Making your check robust
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 Once you've covered your check with the basic "happy path" scenarios, you'll want to
 torture your check with as many edge cases as you can cover in order to ensure your
 check is robust.  Running your check on a large code base, such as Clang/LLVM, is a
 good way to catch things you forgot to account for in your matchers.  However, the
 LLVM code base may be insufficient for testing purposes as it was developed against a