WSL: modcc unit test fails

[brenthuisman]

Running the modcc unit tests on a Win10+WSL+Ubuntu 20.04 build has 1 test failing (all other tests are OK):

/home/user/arbor/test/unit-modcc/test_parser.cpp:663: Failure
The difference between eval(e.get()) and test_case.second is 7.4505805969238281e-09, which exceeds 1e-10, where
eval(e.get()) evaluates to -63281459.031249993,
test_case.second evaluates to -63281459.03125, and
1e-10 evaluates to 1e-10.

A bug has been filed in the WSL repo: microsoft/WSL#830. long double is assumed to be 80 bit, but is 64 bit in WSL.

Since as of today WSL still installs v1 rather than v2 by default, and this is a minor bug, we can change the test to a relative error, e.g.:
EXPECT_TRUE(near_relative(eval(e.get()), test_case.second, 1e-10));
We can instead not assume assumptions about long double, and change eval() accordingly.