stdio-common: Add 'e' conversion tests for . scanf input [BZ #12701]

stdio-common: Add 'e' conversion tests for . scanf input [BZ #12701]

Verify that . input is rejected by 'e' conversion (and its uppercase
counterpart).  Replace 0e0 input with .0e0 rather than adding new one,
because 0 significand is already covered by 0e+0 data, so there's no
need to keep this duplication.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>