unittests: use property for rarely used directories

This avoids having to set them up in cases we don't use them, or
override them in cases where we change the build directory

unittests/baseplatformtests.py

@@ -102,12 +102,18 @@ def setUp(self):
         self.meson_cross_files = []
         self.new_builddir()
 
+    @property
+    def privatedir(self) -> str:
+        return os.path.join(self.builddir, 'meson-private')
+
+    @property
+    def distdir(self) -> str:
+        return os.path.join(self.builddir, 'meson-dist')
+
     def change_builddir(self, newdir):
         self.builddir = newdir
-        self.privatedir = os.path.join(self.builddir, 'meson-private')
         self.logdir = os.path.join(self.builddir, 'meson-logs')
         self.installdir = os.path.join(self.builddir, 'install')
-        self.distdir = os.path.join(self.builddir, 'meson-dist')
         self.mtest_command = self.meson_command + ['test', '-C', self.builddir]
         if os.path.islink(newdir):
             self.addCleanup(os.unlink, self.builddir)
@@ -199,7 +205,6 @@ def init(self, srcdir, *,
                 args += ['--native-file', f]
             for f in self.meson_cross_files:
                 args += ['--cross-file', f]
-        self.privatedir = os.path.join(self.builddir, 'meson-private')
         if inprocess:
             try:
                 returncode, out, err = run_configure_inprocess(['setup'] + self.meson_args + args + extra_args + build_and_src_dir_args, override_envvars)