Make git ignore the file dumped by testFFT.

.gitignore

@@ -70,6 +70,7 @@ tests/test_nonsym_density_1_vec.dat
 tests/test_nonsym_density_2_vec.dat
 tests/test_nonsym_force_1_vec.dat
 tests/test_nonsym_force_2_vec.dat
+tests/potential.dat
 tests/testGreetings
 tests/testReading
 tests/testSingle
@@ -287,4 +288,4 @@ sympy-plots-for-*.tex/
 *.xdy
 
 # macOS
-*.DS_Store
\ No newline at end of file
+*.DS_Store