Fix building the conformance and unit tests on mingw32

The unit tests were failing to build with mingw32 because the
libtest-fixtures was being included both by libcogl and the
test-conformance and test-unit executables. That meant that the
symbols were defined twice and it wouldn't link.

The rule which depends on test-unit being built needs to be called
test-unit.exe on Windows so this patch fixes it to use the $(EXEEXT)
variable to get the right name.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
Fix generating the unit test wrappers when building with MingW32

For some reason the unit test symbols end up in the read-only data
section when building on windows rather than the initialised data
section so they have a different letter in the list generated by nm.
They also begin with an underscore because windows likes to add
underscores to symbols for some reason. This patch changes the regular
expressions in the code to generate the wrapper list so that it
accepts either way.

Reviewed-by: Robert Bragg <robert@linux.intel.com>