tests: Add -mno-relax to link flags for server test

tests: Add -mno-relax to link flags for server test

The flag is required to prevent the test binary from using the gp register
on RISC-V to access global variables. RISC-V ABI assumes the value of the register
is assigned once at sartup (see load_gp() in sysdeps/riscv/start.S in
glibc sources). During tests this code is called for the second time when
app2sd-server-test uses dlopen() to load app2sd-server. Therefore with
relaxed linking enabled for both binaries, the value set at the start
of app2sd-server-test gets overwriten and the test process crashes in
semi-random places.

Change-Id: I6c5d550340c222bb53f8affd0d0c188cc5311765
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>