test: keep coverage reports after coverage-clean

Add coverage folder to .gitignore and remove it from the list
of files & folders delete by coverage-clean.

PR-URL: #15470
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

.gitignore

@@ -36,6 +36,7 @@ node_g
 icu_config.gypi
 .eslintcache
 node_trace.*.log
+coverage/
 
 /out
 

Makefile

@@ -132,7 +132,7 @@ coverage-clean:
 	$(RM) -r node_modules
 	$(RM) -r gcovr testing
 	$(RM) -r out/$(BUILDTYPE)/.coverage
-	$(RM) -r .cov_tmp coverage
+	$(RM) -r .cov_tmp
 	$(RM) out/$(BUILDTYPE)/obj.target/node/{src,gen}/*.gcda
 	$(RM) out/$(BUILDTYPE)/obj.target/node/src/tracing/*.gcda
 	$(RM) out/$(BUILDTYPE)/obj.target/node/{src,gen}/*.gcno