tests/net/gcoap_fileserver: disable test on CI

Recently this test is failing pretty often.
As there are currently no resources to debug this, disable the test to
keep CI running.

tests/net/gcoap_fileserver/Makefile

@@ -21,8 +21,12 @@ USEMODULE += vfs_auto_format
 USEMODULE += hashes
 USEMODULE += shell_cmd_md5sum
 
+# This integration test uncovers a bug somwehere in our stack, but there
+# are currently no resources to debug it, so blacklist it to keep CI running
+TEST_ON_CI_BLACKLIST += all
+
 # automated test only works on native
-TEST_ON_CI_WHITELIST += native
+# TEST_ON_CI_WHITELIST += native
 
 # use small blocksize for test to increase chance for errors
 CFLAGS += -DCONFIG_NANOCOAP_BLOCKSIZE_DEFAULT=COAP_BLOCKSIZE_16