Don't hardcode the filename in CxxExceptionDeathTest.PrintsMessageForStdException

Due to some caveats in the FreeBSD build system and the fact that the
source file is used to compile 2 different death tests with different
flags, I needed (as a shortterm workaround) to copy the test to 2
differently named files.

While this works for compiling the test, as I discovered, this doesn't
work with running CxxExceptionDeathTest.PrintsMessageForStdException,
as the testcase hardcodes googletest-death-test_ex_test.cc. Use __FILE__
when looking for failures, as opposed to looking for the hardcoded name
as it can vary depending on how the test was built.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>