Fix GCC-8 warnings in the test suite.

tests/test125cells.c

@@ -470,7 +470,7 @@ int main(int argc, char *argv[]) {
   size_t runs = 0, particles = 0;
   double h = 1.23485, size = 1., rho = 2.5;
   double perturbation = 0.;
-  char outputFileNameExtension[200] = "";
+  char outputFileNameExtension[100] = "";
   char outputFileName[200] = "";
   enum velocity_field vel = velocity_zero;
   enum pressure_field press = pressure_const;

tests/test27cells.c

@@ -342,7 +342,7 @@ int main(int argc, char *argv[]) {
   size_t runs = 0, particles = 0;
   double h = 1.23485, size = 1., rho = 1.;
   double perturbation = 0., h_pert = 0.;
-  char outputFileNameExtension[200] = "";
+  char outputFileNameExtension[100] = "";
   char outputFileName[200] = "";
   enum velocity_types vel = velocity_zero;
 

tests/testActivePair.c

@@ -429,7 +429,7 @@ int main(int argc, char *argv[]) {
   struct runner *runner;
   char c;
   static long long partId = 0;
-  char outputFileNameExtension[200] = "";
+  char outputFileNameExtension[100] = "";
   char swiftOutputFileName[200] = "";
   char bruteForceOutputFileName[200] = "";
 

tests/testPeriodicBC.c

@@ -388,7 +388,7 @@ int main(int argc, char *argv[]) {
   double h = 1.23485, size = 1., rho = 1.;
   double perturbation = 0.;
   double threshold = ACC_THRESHOLD;
-  char outputFileNameExtension[200] = "";
+  char outputFileNameExtension[100] = "";
   char swiftOutputFileName[200] = "";
   char bruteForceOutputFileName[200] = "";
   enum velocity_types vel = velocity_zero;