Restore bitshift.exp tests

Restore bitshift.exp tests

Commit cdd4206647 unintentionally disabled all tests of bitshift.exp,
so it actually just does this:

Running /c/src/repos/binutils-gdb.git/gdb/testsuite/gdb.base/bitshift.exp ...
PASS: gdb.base/bitshift.exp: complete set language

		=== gdb Summary ===

 # of expected passes		1

It changed the 'continue' of unsupported languages to 'return', and
since ada is the first language and is unsupported, no tests were run.

This changes it back to 'continue', and the following patches fix
the regressions that were introduced since then unnoticed.

Approved-By: Tom Tromey <tom@tromey.com>