tests/gcoap_fileserver: only enable test with GCC

We see a lot of failures on this on CI with LLVM.
Blacklist the LLVM build until someone manages to find out what
the problem is.

tests/net/gcoap_fileserver/Makefile

@@ -23,6 +23,8 @@ USEMODULE += shell_cmd_md5sum
 
 # automated test only works on native
 TEST_ON_CI_WHITELIST += native
+# FIXME: for some reason the test fails very often when built with clang
+TOOLCHAINS_BLACKLIST += llvm
 
 # use small blocksize for test to increase chance for errors
 CFLAGS += -DCONFIG_NANOCOAP_BLOCKSIZE_DEFAULT=COAP_BLOCKSIZE_16