test: build wide shifts from a variable so the file still compiles - #7117

An integer literal that does not fit `mrb_int` is rejected while the file
is compiled, not when the line runs, so `1 << 63` in a test drops the
whole test file on a build with a 32-bit `mrb_int` and no bigint. Nothing
reports it: the tests are not skipped and not failed, they are simply
absent, and the suite goes green with fewer tests than it had.

Three test files pay for this today. Counted on a build with `MRB_INT32`
and no bigint:

* `test/t/string.rb`, from `~(-1 << 63)`: 71 tests
* `mrbgems/mruby-random/test/random.rb`, from `1 << 62`: 13 tests
* `mrbgems/mruby-socket/test/addrinfo.rb`, from `1 << 40`: 10 tests

All three already meant to handle a narrow `mrb_int`. The `rescue nil` in
mruby-random and the `skip` in mruby-socket never got the chance to run,
and the `begin`/`rescue` in `String#bytesplice` covers a line the compiler
had already thrown away.

Take the shift width from a variable in each of them. The shift is then a
run time operation that raises `RangeError` where `mrb_int` is too narrow,
which is what the surrounding `rescue` and `skip` were written for.

`Kernel#rand integer range overflow` reaches its wide bounds only where
`mrb_int` is 64 bits wide, and it decided that by calling `rand` on the very
range whose overflow it guards against. The regression it covers returns `nil`
for that range, which the guard read as "this build has no integer range path",
so every assertion behind it was skipped and the suite stayed green with the
bug in place.

Ask what the bounds are instead. `1 << 62` raises `RangeError` where `mrb_int`
is 32 bits and mruby-bigint is absent, and where the gem promotes it the result
is a big integer, which a core method taking an `mrb_int` rejects with the same
error. Neither kind of bound reaches `rand_range_int()`, which is the path the
assertions are written for.

Checked by putting the signed `end - begin` back into `rand_range_int()`: the
wide range assertions now fail there instead of being skipped.