Run unit tests under valgrind.

Run a separate build for the unit tests under Valgrind.  They take long
enough that running in parallel with the other Valgrind tests helps.

.github/configs

@@ -84,6 +84,9 @@ case "$config" in
 	    valgrind-4)
 		LTESTS="${tests4}"
 		;;
+	    valgrind-unit)
+		TEST_TARGET="unit USE_VALGRIND=1"
+		;;
 	esac
 	;;
     *)

.github/workflows/c-cpp.yml

@@ -22,6 +22,7 @@ jobs:
           - { os: ubuntu-20.04, configs: valgrind-2 }
           - { os: ubuntu-20.04, configs: valgrind-3 }
           - { os: ubuntu-20.04, configs: valgrind-4 }
+          - { os: ubuntu-20.04, configs: valgrind-unit }
           - { os: ubuntu-20.04, configs: pam }
           - { os: ubuntu-20.04, configs: kitchensink }
           - { os: ubuntu-20.04, configs: hardenedmalloc }