Fixes for tests on i386: relax SphericalCoordinates and workaround for negative zero

[j-rivero]

Two changes to make i386 testing build happy after ign-math 6.10:

• OrientedBox_TEST: the problem appeared before, when comparing strings that comes from a zero value in maths, the optimizations in some platforms return a negative zero making the comparison to fail. The workaround is to compare the two substrings around the zero.
• SphericalCoordinates_TEST: some float operations in i386 does not return the exact number but a really close one. Change the test to reflect this reality.

[codecov]

Codecov Report

Merging #374 (908fcc1) into ign-math6 (3b3e7ab) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #374   +/-   ##
==========================================
  Coverage      99.65%   99.65%           
==========================================
  Files             67       67           
  Lines           6380     6380           
==========================================
  Hits            6358     6358           
  Misses            22       22           

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[mjcarroll]

One comment to maybe make this an easier fix.