docs: document test commands (#3816)

So that contributors can quickly figure out how to run tests.

Signed-off-by: Will Murphy <willmurphyscode@users.noreply.github.com>

DEVELOPING.md

@@ -227,6 +227,16 @@ rough outline how that works:
 
 ## Testing
 
+### Testing commands
+
+* `make help` shows a list of available commands
+* `make unit`, `make integration`, `make cli`, and `make acceptance` run those test suites (see below)
+* `make test` runs all those tests (and is therefore pretty slow)
+* `make fixtures` clears and re-fetches all test fixtures.
+* `go test ./syft/pkg/` for example can test particular packages, assuming fixtures are already made
+* `make clean-cache` cleans all test cache. Note that subsequent test runs will be slower after this
+
+
 ### Levels of testing
 
 - `unit`: The default level of test which is distributed throughout the repo are unit tests. Any `_test.go` file that 

Taskfile.yaml

@@ -224,7 +224,7 @@ tasks:
       - "go run -race cmd/syft/main.go alpine:latest"
 
   validate-cyclonedx-schema:
-    desc: Run integration tests
+    desc: Validate that Syft produces valid CycloneDX documents
     cmds:
       - "cd schema/cyclonedx && make"