Fix test failure due to Num stringification

[treyharris]

Currently, you get a test failure like

===> Testing: Perl6-Math-Quaternion:auth<Bruce Gray>
[Perl6-Math-Quaternion] # Failed test '.norm'
[Perl6-Math-Quaternion] # at t/math_ops.t line 19
[Perl6-Math-Quaternion] # expected: '5.47722557505166'
[Perl6-Math-Quaternion] #      got: '5.477225575051661'
[Perl6-Math-Quaternion] # Looks like you failed 1 test of 38
===> Testing [FAIL]: Perl6-Math-Quaternion:auth<Bruce Gray>

Just change the is $q.norm, 5.47722557505166, '.norm' to use
is-aprox instead.

[treyharris]

If you’re receiving notifications via email, you’ll need to re-read this PR on the web at #9

Sorry for the initial truncated pull-req message; I had a sticky modifier key I didn’t realize and submitted the PR when I was trying to edit the message.

[AlexDaniel]

Same as #7, can be closed.