Exclude /usr and deps from coverage reports

fastcov was including system headers (/usr/include/c++/...) and vendored
dependencies (deps/json/json.hpp) in coverage output. Use --exclude
instead of --include since project source files have bare filenames in
gcov output (no path prefix to match against), while system/deps paths
are always absolute.

test/infra/control/run-multi-group.bash

@@ -360,6 +360,7 @@ if [ "${COVERAGE}" -eq 1 ]; then
                     if command -v fastcov >/dev/null 2>&1; then
                         cd "${GCOV_DIR}"
                         fastcov -b -j4 -l \
+                            -e /usr deps \
                             -d . -o "${GROUP_INFO}" >> "${COVERAGE_LOG}" 2>&1 || \
                             echo ">>> WARNING: fastcov failed for ${GCOV_DIR}" >> "${COVERAGE_LOG}"
                     fi

test/infra/control/run-tests-isolated.bash

@@ -312,6 +312,7 @@ docker run \
                             echo \">>> Running fastcov on /gcov...\"
                             cd /gcov
                             fastcov -b -j\$(nproc) -l \
+                                -e /usr deps \
                                 -d . -o \"\${coverage_file}\" >> \"\${coverage_log}\" 2>&1 || echo \">>> WARNING: Coverage generation failed (see \${coverage_log})\"
 
                             if [ -f \"\${coverage_file}\" ]; then