tests: use BUILT_SOURCES in tests/Makefile.am for automake 1.16.1

tests: use BUILT_SOURCES in tests/Makefile.am for automake 1.16.1

When trying to run make check using automake 1.16.1, we get:

CC isisd/test_fuzz_isis_tlv-test_fuzz_isis_tlv.o
isisd/test_fuzz_isis_tlv.c:1:10: fatal error: test_fuzz_isis_tlv_tests.h: No such file or directory
^~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
Makefile:1096: recipe for target 'isisd/test_fuzz_isis_tlv-test_fuzz_isis_tlv.o' failed
make[1]: *** [isisd/test_fuzz_isis_tlv-test_fuzz_isis_tlv.o] Error 1
make[1]: Leaving directory '/src/frr-frr-5.0/tests'
Makefile:1220: recipe for target 'check-am' failed
make: *** [check-am] Error 2

From reading the automake docs, it looks like there may be a more
reliable way to express built files in the Makefile.am using BUILT_SOURCES.
Using this method, we seem to build fine now on 1.16.1 and this
has been tested on Ubuntu 18.04, CentOS 7 and Alpine edge (which uses
automake 1.16.1).

Issue: https://github.com/FRRouting/frr/issues/2403
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>