Grains test doesn't handle bugs caused by `**`

[sshine]

This should fail the test suite:

sub total_grains {
    return 2 ** 64 - 42;
}

The test appears to pass for 2 ** 64, 2 ** 64 - 1 and 2 ** 64 - 2, which can't all be true.

This is because ** is made with C's pow which operates internally on doubles. The current test converts the expected value 18446744073709551615 to a double and an imprecise comparison erroneously succeeds.

The test suite should detect this and report a problem to relieve mentors of having to point it out.

Perhaps the test suite can provide a negative test,

unlike total_grains(), qr/e\+/, "Using '**' without 'use bignum;' uses doubles that are too imprecise for this result.";

Additionally, use 'eq' instead of '==' to compare the result as strings instead of numbers, since then the error message shows up as:

#   Failed test 'returns the total number of grains on the board'
#   at ./grains.t line 16.
#          got: '1.84467440737096e+19'
#     expected: '18446744073709551615'

which should more clearly indicate the problem that the lower digits have vanished.

Perhaps apply this reasoning in the test for grains_on_square 64, since otherwise ** erroneously succeeds in this case, too:

#   Failed test 'square no. 64'
#   at ./grains.t line 23.
#          got: '9.22337203685478e+18'
#     expected: '9223372036854775808'

[kappa]

This is still broken and students stumble upon it.