[gdb/testsuite] Fix test name in gdb.python/python.exp

[gdb/testsuite] Fix test name in gdb.python/python.exp

When running test-case gdb.python/python.exp, we have:
...
PASS: gdb.python/python.exp: starti via gdb.execute, not from tty
PASS: gdb.python/python.exp: starti via interactive input
...

The two tests are instances of the same test, with different values for
starti command argument from_tty, so it's strange that the test names are so
different.

This is due to using a gdb_test nested in a gdb_test_multiple, with the inner
one using a different test name than the outer one.  [ That could still make
sense if both produced passes, but that's not the case here. ]

Fix this by using $gdb_test_name, such that we have:
...
PASS: gdb.python/python.exp: starti via gdb.execute, not from tty
PASS: gdb.python/python.exp: starti via gdb.execute, from tty
...

Also make this more readable by using variables.

Tested on x86_64-linux.