Use environment variables for test data base paths (#13474)

## Feat
Add optional env variables for test data base paths so devs can set it to a local path without running the risk of committing changes that shouldn't be there.

## Reason
I did exactly that and want to add the env variable to my `.bashrc`. No more timeouts from github and faster tests!

Author: matthdsm

tests/config/nf-test.config

@@ -1,8 +1,8 @@
 params {
     publish_dir_mode                  = "copy"
     singularity_pull_docker_container = false
-    test_data_base                    = 'https://raw.githubusercontent.com/nf-core/test-datasets/modules'
-    modules_testdata_base_path        = 'https://raw.githubusercontent.com/nf-core/test-datasets/modules/data/'
+    test_data_base                    = System.getenv('NXF_TEST_DATA_BASE') ?: 'https://raw.githubusercontent.com/nf-core/test-datasets/modules'
+    modules_testdata_base_path        = System.getenv('NXF_MODULES_TESTDATA_BASE') ?: 'https://raw.githubusercontent.com/nf-core/test-datasets/modules/data/'
 }
 
 // Resolves issue with accessing s3 from self-hosted runners