gdb/testsuite: resolve duplicate test names in gdb.threads/tls.exp

gdb/testsuite: resolve duplicate test names in gdb.threads/tls.exp

While running the gdb.threads/tls.exp test with a GDB configured
without Python, I noticed some duplicate test names.

This is caused by a call to skip_python_tests that is within a proc
that is called multiple times by the test script.  Each call to
skip_python_tests results in a call to 'unsupported', and this causes
the duplicate test names.

After this commit we now call skip_python_tests just once and place
the result into a variable.  Now, instead of calling skip_python_tests
multiple times, we just check the variable.

There should be no change in what is tested after this commit.