tests/docker: only enable ubsan for test-clang

-fsanitize=undefined is not the same thing as --enable-sanitizers.  After
commit 47c823e ("tests/docker: add sanitizers back to clang build", 2019-09-11)
test-clang is almost duplicating the asan (test-debug) test, so
partly revert commit 47c823e5b while leaving ubsan enabled.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/docker/test-clang

@@ -17,7 +17,9 @@ requires clang
 
 cd "$BUILD_DIR"
 
-OPTS="--cxx=clang++ --cc=clang --host-cc=clang --enable-sanitizers"
+OPTS="--cxx=clang++ --cc=clang --host-cc=clang"
+OPTS="$OPTS --extra-cflags=-fsanitize=undefined \
+    --extra-cflags=-fno-sanitize=float-divide-by-zero"
 build_qemu $OPTS
 check_qemu
 install_qemu