Fix floating-point string compare in CoverageTest.

Because 1.2f has no exact floating point representation, this test fails with -draftCompile and -ea.  Using 1.5f which can be represented exactly.

http://gwt-code-reviews.appspot.com/312804/show
Suggested by: spoon
Review by: tobyr


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7942 8db76d5a-ed1c-0410-87a9-c151d255dfc7