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GH-35606: [CI][C++][MinGW32] Use more accurate float inputs for decimal test (#35680)

### Rationale for this change

If we use `4.611686e+18f` for `2**62` value, MinGW 32bit can't generate enough precious double value.
See the following errors for this:

```text
[----------] 2 tests from TestDecimalFromReal/1, where TypeParam = std::pair<arrow::Decimal128, double>
[ RUN      ] TestDecimalFromReal/1.TestSuccess
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "4611686000000000000"
  expected
    Which is: "4611686018427387904"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-4611686000000000000"
  expected
    Which is: "-4611686018427387904"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "9223372000000000000"
  expected
    Which is: "9223372036854775808"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-9223372000000000000"
  expected
    Which is: "-9223372036854775808"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "18446744000000000000"
  expected
    Which is: "18446744073709551616"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-18446744000000000000"
  expected
    Which is: "-18446744073709551616"
[  FAILED  ] TestDecimalFromReal/1.TestSuccess, where TypeParam = std::pair<arrow::Decimal128, double> (0 ms)
[ RUN      ] TestDecimalFromReal/1.TestErrors
[       OK ] TestDecimalFromReal/1.TestErrors (0 ms)
[----------] 2 tests from TestDecimalFromReal/1 (0 ms total)

[----------] 2 tests from TestDecimalFromReal/2, where TypeParam = std::pair<arrow::Decimal256, float>
[ RUN      ] TestDecimalFromReal/2.TestSuccess
[       OK ] TestDecimalFromReal/2.TestSuccess (0 ms)
[ RUN      ] TestDecimalFromReal/2.TestErrors
[       OK ] TestDecimalFromReal/2.TestErrors (0 ms)
[----------] 2 tests from TestDecimalFromReal/2 (0 ms total)

[----------] 2 tests from TestDecimalFromReal/3, where TypeParam = std::pair<arrow::Decimal256, double>
[ RUN      ] TestDecimalFromReal/3.TestSuccess
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "4611686000000000000"
  expected
    Which is: "4611686018427387904"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-4611686000000000000"
  expected
    Which is: "-4611686018427387904"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "9223372000000000000"
  expected
    Which is: "9223372036854775808"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-9223372000000000000"
  expected
    Which is: "-9223372036854775808"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "18446744000000000000"
  expected
    Which is: "18446744073709551616"
D:/a/arrow/arrow/cpp/src/arrow/util/decimal_test.cc:766: Failure
Expected equality of these values:
  dec.ToString(scale)
    Which is: "-18446744000000000000"
  expected
    Which is: "-18446744073709551616"
[  FAILED  ] TestDecimalFromReal/3.TestSuccess, where TypeParam = std::pair<arrow::Decimal256, double> (0 ms)
[ RUN      ] TestDecimalFromReal/3.TestErrors
```

### What changes are included in this PR?

This changes use `4.6116860184273879e+18` for `2**62` value. It works with MinGW 32bit too.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #35606

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>