Tests: Quote possibly empty string(REPLACE) argument

The ${unity_source_line} variable should not normally be empty,
but it has been observed that it sometimes is due to other factors.
Rather than causing string(REPLACE) to fail due to the wrong
number of arguments, quote the variable so that the
UnityBuildPre2017 test case fails with a more appropriate
error in the block just after the usage fixed by this change.
1 file changed