tests docs: fix lcov code coverage report generation

- Updated/rebased version of https://gerrit.fd.io/r/c/vpp/+/34199

Type: test
Change-Id: I43913ecfd11a4578bdb10c4be76253fe38d57976
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>

docs/developer/build-run-debug/code_coverage.rst

@@ -0,0 +1,45 @@
+.. _lcov_code_coverage:
+
+Code coverage with lcov
+=======================
+
+Prerequisites
+-------------
+
+Ensure required packages are installed:
+
+::
+
+    $ make install-deps
+
+The Linux gcov and lcov tools can be fussy about gcc / g++ compiler
+versions. As of this writing, Ubuntu 22.04 gcov / lcov works with
+the latest gcc version (``11.3.0``).
+
+Generate coverage for a test case
+---------------------------------
+
+As a first run, in order to generate the coverage report of
+a specific plugin or test, run for example
+
+::
+
+    $ make test-cov TEST=fib
+
+Then open the file ``.build-root/test-coverage/html/index.html`` in a Chrome browser.
+
+Improving test coverage
+-----------------------
+
+When doing modifications on the test cases, you can run
+
+::
+
+    # This will run the test & report the result in the coverage data
+    $ make test-cov-build TEST=fib
+
+    # This will generate the html report with the current coverage data
+    $ make test-cov-post
+
+    # To reset the coverage data use
+    $ make test-cov-prep

docs/developer/build-run-debug/index.rst

@@ -12,3 +12,4 @@ Build, Run & Debug
    testing_vpp
    gdb_examples
    cross_compile_macos
+   code_coverage