NumberToStringGTest: add #include, avoid narrowing conversion pow, mention floating point type on an expect failure

[N-Dekker]

A few small improvements of NumberToStringGTest, triggered by the NumberToString.DecimalNotationUpTo21Digits failures reported by @yurivict at issue #3739 saying:

/disk-samsung/freebsd-ports/science/InsightToolkit/work/ITK-5.2.1/Modules/Core/Common/test/itkNumberToStringGTest.cxx:87: Failure
Expected equality of these values:
  numberToString(power_of_ten)
    Which is: "100000010000"
  '1' + std::string(exponent, '0')
    Which is: "100000000000"

[N-Dekker]

Thanks for your approval @dzenanz. Before merging please let's wait for @yurivict to check if it really fixes the NumberToStringGTest failures at FreeBSD 13.1/clang-14 (issue #3739). (Otherwise this is still a nice STYLE PR, IMhO.)

[yurivict]

This PR does not resolve the failure.

382/4247 Testing: NumberToString.DecimalNotationUpTo21Digits
382/4247 Test: NumberToString.DecimalNotationUpTo21Digits
Command: "/disk-samsung/freebsd-ports/science/InsightToolkit/work/.build/bin/ITKCommonGTestDriver" "--gtest_filter=NumberToString.DecimalNotationUpTo21Digits" "--gtest_also_run_disabled_tests"
Directory: /disk-samsung/freebsd-ports/science/InsightToolkit/work/.build/Modules/Core/Common/test
"NumberToString.DecimalNotationUpTo21Digits" start time: Nov 19 17:42 PST
Output:
----------------------------------------------------------
Running main() from /wrkdirs/usr/ports/devel/googletest/work/googletest-release-1.12.1/googletest/src/gtest_main.cc
Note: Google Test filter = NumberToString.DecimalNotationUpTo21Digits
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from NumberToString
[ RUN      ] NumberToString.DecimalNotationUpTo21Digits
/disk-samsung/freebsd-ports/science/InsightToolkit/work/ITK-5.3rc04/Modules/Core/Common/test/itkNumberToStringGTest.cxx:88: Failure
Expected equality of these values:
  numberToString(power_of_ten)
    Which is: "100000010000"
  '1' + std::string(exponent, '0')
    Which is: "100000000000"
/disk-samsung/freebsd-ports/science/InsightToolkit/work/ITK-5.3rc04/Modules/Core/Common/test/itkNumberToStringGTest.cxx:89: Failure
Expected equality of these values:
  numberToString(-power_of_ten)
    Which is: "-100000010000"
  "-1" + std::string(exponent, '0')
    Which is: "-100000000000"
/disk-samsung/freebsd-ports/science/InsightToolkit/work/ITK-5.3rc04/Modules/Core/Common/test/itkNumberToStringGTest.cxx:97: Failure
Expected equality of these values:
  numberToString(power_of_ten)
    Which is: "0.000009999999999999999"
  "0." + std::string(-1 - exponent, '0') + '1'
    Which is: "0.00001"
/disk-samsung/freebsd-ports/science/InsightToolkit/work/ITK-5.3rc04/Modules/Core/Common/test/itkNumberToStringGTest.cxx:98: Failure
Expected equality of these values:
  numberToString(-power_of_ten)
    Which is: "-0.000009999999999999999"
  "-0." + std::string(-1 - exponent, '0') + '1'
    Which is: "-0.00001"
[  FAILED  ] NumberToString.DecimalNotationUpTo21Digits (0 ms)
[----------] 1 test from NumberToString (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] NumberToString.DecimalNotationUpTo21Digits
 
 1 FAILED TEST

[N-Dekker]

Thanks @yurivict I removed the claim that this PR would fix those NumberToString.DecimalNotationUpTo21Digits failures 😞

But I also added an extra commit, with should make the test failures slightly more informative, by mentioning the floating point type ("float" or "double") with each expectation failure. Can you please try that out as well?