Speed up Helpers::Pair test

[mjcarroll]

Housekeeping on the math::Pair unittest.

I'm not sure that exhaustively testing this many pairs is worthwhile, in general.

On my system (before):

[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from HelpersTest
[ RUN      ] HelpersTest.Pair
Tested: 25048098 keys 
[       OK ] HelpersTest.Pair (5872 ms)
[----------] 1 test from HelpersTest (5872 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (5872 ms total)
[  PASSED  ] 1 test.
./bin/UNIT_Helpers_TEST --gtest_filter="*.Pair"  5.66s user 0.26s system 99% cpu 5.935 total

(after):

[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from HelpersTest
[ RUN      ] HelpersTest.Pair
Tested: 46829 keys
.[       OK ] HelpersTest.Pair (9 ms)
[----------] 1 test from HelpersTest (9 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (9 ms total)
[  PASSED  ] 1 test.
./bin/UNIT_Helpers_TEST --gtest_filter="*.Pair"  0.01s user 0.00s system 96% cpu 0.016 total

I believe that @nkoenig and @scpeters were the ones who most recently touched this function, so pinging them for reviews.

Signed-off-by: Michael Carroll [email protected]

Note to maintainers: Remember to use Squash-Merge

[mjcarroll]

@osrf-jenkins retest this please