meson: do not attempt to install tests when they are disabled

meson: do not attempt to install tests when they are disabled

If -Dtests=false but -Dinstall-tests=true the build will fail, as some tests will
be pulled in the build but not their prerequisites. It doesn't make sense to ask
for tests to be installed if they are disabled.

FAILED: test-acd
cc  -o test-acd test-acd.p/src_libsystemd-network_test-acd.c.o -flto -Wl,--as-needed -Wl,--no-undefined -pie -fstack-protector -Wl,-z,relro -specs=/usr/share/debhelper/dh_package_notes/debian-package-notes.specs -g -O2 -ffile-prefix-map=/tmp/s=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -ffat-lto-objects -Wdate-time -D_FORTIFY_SOURCE=2 '-Wl,-rpath,$ORIGIN/src/shared:XXXXXXXXXXXXXXX' -Wl,-rpath-link,/tmp/s/obj-x86_64-linux-gnu/src/shared -Wl,--start-group src/shared/libsystemd-shared-255.so src/libsystemd-network/libsystemd-network.a -Wl,--end-group -Wl,--fatal-warnings -Wl,-z,now -Wl,-z,relro -Wl,--warn-common -Wl,--gc-sections -Wl,--fatal-warnings -Wl,-z,now -Wl,-z,relro -Wl,--warn-common -Wl,--gc-sections
/usr/bin/ld: /tmp/cc0oYwFZ.ltrans0.ltrans.o: in function `main':
./obj-x86_64-linux-gnu/./obj-x86_64-linux-gnu/<artificial>:85:(.text.startup+0x33): undefined reference to `test_setup_logging'
collect2: error: ld returned 1 exit status