run_unittests.sh: fix Python DeprecationWarning

Fix the following Python error:

  D:\a\1\s\run_unittests.py:6654: DeprecationWarning: invalid escape sequence \
    self.assertEqual(libhello_nolib.get_pkgconfig_variable(escaped_var, {}), hello world)

Use a raw string literal.

run_unittests.py

@@ -6651,7 +6651,7 @@ class LinuxlikeTests(BasePlatformTests):
         self.assertEqual(libhello_nolib.get_compile_args(), [])
         self.assertEqual(libhello_nolib.get_pkgconfig_variable('foo', {}), 'bar')
         self.assertEqual(libhello_nolib.get_pkgconfig_variable('prefix', {}), self.prefix)
-        self.assertEqual(libhello_nolib.get_pkgconfig_variable('escaped_var', {}), 'hello\ world')
+        self.assertEqual(libhello_nolib.get_pkgconfig_variable('escaped_var', {}), r'hello\ world')
         self.assertEqual(libhello_nolib.get_pkgconfig_variable('unescaped_var', {}), 'hello world')
 
         cc = env.detect_c_compiler(MachineChoice.HOST)