Fix missing base filename for check-modem300 test

[mfncooper]

The base filename for the check-modem300 test was missing, probably due to a copy / paste error, causing the test to be skipped when GitHub ran its build / test action.

[wb2osz]

Thanks for finding this.
Why did it work for Linux but not Windows?

[mfncooper]

Thanks for merging.

The reason it worked on Linux and Mac is because CUSTOM_SCRIPT_SUFFIX is an empty string on those platforms, which meant that the output value for configure_file(), with a missing file name, was actually a directory rather than a file name. Since we preserve the file name in the copy, the result was correct, even if by accident. On Windows, CUSTOM_SCRIPT_SUFFIX is ".bat", so configure_file() was copying the script to a file named ".bat", instead of "check-modem300.bat". Later, in add_test(), the correct file name was being used, and that file did not exist because of the incorrect ".bat" target of configure_file().