[gdb/testsuite] Allow gdb.rust/expr.exp without rust compiler

Proc allow_rust_tests returns 0 when there's no rust compiler, but that gives
the wrong answer for gdb.rust/expr.exp, which doesn't require it.

Fix this by using can_compile rust in the test-cases that need it, and just
returning 1 in allow_rust_tests.

Tested on x86_64-linux.