Merge #19916: build: allow user to specify DIR_FUZZ_SEED_CORPUS for cov_fuzz

fb3bacc .gitignore: ignore qa-assets/ folder (eugene)
a9f2014 build: use DIR_FUZZ_SEED_CORPUS if specified for cov_fuzz target (eugene)

Pull request description:

  This PR contains two commits:
  - The cov_fuzz target now uses `DIR_FUZZ_SEED_CORPUS` as the seed directory instead of the hard-coded `qa-assets/fuzz_seed_corpus`. Otherwise, running it requires me to copy the corpus to the bitcoin directory first. In case `DIR_FUZZ_SEED_CORPUS` is not specified, the original default is used.
  - add qa-assets folder to gitignore

  Example usage:
  `make cov_fuzz DIR_FUZZ_SEED_CORPUS=~/workspace/qa-assets/fuzz_seed_corpus`

  It can also just be an environment variable.

ACKs for top commit:
  fanquake:
    ACK fb3bacc - looks fine.
  practicalswift:
    ACK fb3bacc - patch looks correct

Tree-SHA512: 19ad7c6a2b0c088df14fb71a217d956e66a69eea78f016cd1e914d39c5d7cc196766a637e6c941c6706322663010e7162e85f57e888b8f3b05d0c37d44740847

Author: fanquake

.gitignore

@@ -122,6 +122,7 @@ total.coverage/
 fuzz.coverage/
 coverage_percent.txt
 /cov_tool_wrapper.sh
+qa-assets/
 
 #build tests
 linux-coverage-build

Makefile.am

@@ -193,6 +193,8 @@ LCOV_FILTER_PATTERN = \
 	-p "src/secp256k1" \
 	-p "depends"
 
+DIR_FUZZ_SEED_CORPUS ?= qa-assets/fuzz_seed_corpus
+
 $(COV_TOOL_WRAPPER):
 	@echo 'exec $(COV_TOOL) "$$@"' > $(COV_TOOL_WRAPPER)
 	@chmod +x $(COV_TOOL_WRAPPER)
@@ -205,7 +207,7 @@ baseline_filtered.info: baseline.info
 	$(LCOV) -a $@ $(LCOV_OPTS) -o $@
 
 fuzz.info: baseline_filtered.info
-	@TIMEOUT=15 test/fuzz/test_runner.py qa-assets/fuzz_seed_corpus -l DEBUG
+	@TIMEOUT=15 test/fuzz/test_runner.py $(DIR_FUZZ_SEED_CORPUS) -l DEBUG
 	$(LCOV) -c $(LCOV_OPTS) -d $(abs_builddir)/src --t fuzz-tests -o $@
 	$(LCOV) -z $(LCOV_OPTS) -d $(abs_builddir)/src