meson: use install_symlink() where applicable

Now our baseline of meson is 0.62, hence install_symlink() can be used.

Note, install_symlink() implies install_emptydir() for specified
install_dir. Hence, this also drops several unnecessary
install_emptydir() calls.

Note, the function currently does not support 'relative' and 'force' flags,
so several 'ln -frsT' inline calls cannot be replaced.

test/meson.build

@@ -506,6 +506,8 @@ if install_tests
 
         # The unit tests implemented as shell scripts expect to find testdata/
         # in the directory where they are stored.
-        meson.add_install_script(sh, '-c', ln_s.format(testdata_dir,
-                                                       unittestsdir / 'testdata'))
+        assert(fs.parent(unittestsdir) / 'testdata' == testdata_dir)
+        install_symlink('testdata',
+                        pointing_to : '../testdata',
+                        install_dir : unittestsdir)
 endif