Makefile: allow overriding the packages to test in `make test`

This way you can for example run `make test GOTESTPKGS=./builder` to
only test the builder package.
I've often done this by manually modifying the Makefile, so having a
make parameter available would make this much easier.

GNUmakefile

@@ -31,6 +31,7 @@ export GOROOT = $(shell $(GO) env GOROOT)
 
 # Flags to pass to go test.
 GOTESTFLAGS ?=
+GOTESTPKGS ?= ./builder ./cgo ./compileopts ./compiler ./interp ./transform .
 
 # tinygo binary for tests
 TINYGO ?= $(call detect,tinygo,tinygo $(CURDIR)/build/tinygo)
@@ -283,7 +284,7 @@ tinygo:
 	@if [ ! -f "$(LLVM_BUILDDIR)/bin/llvm-config" ]; then echo "Fetch and build LLVM first by running:"; echo "  $(MAKE) llvm-source"; echo "  $(MAKE) $(LLVM_BUILDDIR)"; exit 1; fi
 	CGO_CPPFLAGS="$(CGO_CPPFLAGS)" CGO_CXXFLAGS="$(CGO_CXXFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" $(GOENVFLAGS) $(GO) build -buildmode exe -o build/tinygo$(EXE) -tags "byollvm osusergo" -ldflags="-X github.com/tinygo-org/tinygo/goenv.GitSha1=`git rev-parse --short HEAD`" .
 test: wasi-libc check-nodejs-version
-	CGO_CPPFLAGS="$(CGO_CPPFLAGS)" CGO_CXXFLAGS="$(CGO_CXXFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" $(GO) test $(GOTESTFLAGS) -timeout=20m -buildmode exe -tags "byollvm osusergo" ./builder ./cgo ./compileopts ./compiler ./interp ./transform .
+	CGO_CPPFLAGS="$(CGO_CPPFLAGS)" CGO_CXXFLAGS="$(CGO_CXXFLAGS)" CGO_LDFLAGS="$(CGO_LDFLAGS)" $(GO) test $(GOTESTFLAGS) -timeout=20m -buildmode exe -tags "byollvm osusergo" $(GOTESTPKGS)
 
 # Standard library packages that pass tests on darwin, linux, wasi, and windows, but take over a minute in wasi
 TEST_PACKAGES_SLOW = \