StatisticsTest.CV: don't require precise FP match, tolerate some abs …

…error

We get ever so slightly different results on windows with GCC.
```
71: Test command: D:\a\benchmark\benchmark\_build\test\statistics_gtest.exe
71: Working Directory: D:/a/benchmark/benchmark/_build/test
71: Test timeout computed to be: 10000000
71: Running main() from gmock_main.cc
71: [==========] Running 4 tests from 1 test suite.
71: [----------] Global test environment set-up.
71: [----------] 4 tests from StatisticsTest
71: [ RUN      ] StatisticsTest.Mean
71: [       OK ] StatisticsTest.Mean (0 ms)
71: [ RUN      ] StatisticsTest.Median
71: [       OK ] StatisticsTest.Median (0 ms)
71: [ RUN      ] StatisticsTest.StdDev
71: [       OK ] StatisticsTest.StdDev (0 ms)
71: [ RUN      ] StatisticsTest.CV
71: D:/a/benchmark/benchmark/test/statistics_gtest.cc:31: Failure
71: Expected equality of these values:
71:   benchmark::StatisticsCV({2.5, 2.4, 3.3, 4.2, 5.1})
71:     Which is: 0.32888184094918088
71:   0.32888184094918121
71: [  FAILED  ] StatisticsTest.CV (0 ms)
71: [----------] 4 tests from StatisticsTest (0 ms total)
```

test/statistics_gtest.cc

@@ -28,8 +28,8 @@ TEST(StatisticsTest, StdDev) {
 TEST(StatisticsTest, CV) {
   EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({101, 101, 101, 101}), 0.0);
   EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({1, 2, 3}), 1. / 2.);
-  EXPECT_DOUBLE_EQ(benchmark::StatisticsCV({2.5, 2.4, 3.3, 4.2, 5.1}),
-                   0.32888184094918121);
+  ASSERT_NEAR(benchmark::StatisticsCV({2.5, 2.4, 3.3, 4.2, 5.1}),
+              0.32888184094918121, 1e-15);
 }
 
 }  // end namespace