While working on reproducible builds for openSUSE, I found that
possibly depending on build host load, the 2D_convergence test sometimes fails.

diff between a good and a bad run:
 Frequency difference with a of 15 is 0.0153091/15/15
-frequency for a=10 is 0.180252, 0 (shifted), 0.0901258 (mean)
+frequency for a=10 is 0.180252, 0.181218 (shifted), 0.180735 (mean)
 Unshifted freq error is 0.0307579/10/10
-Shifted freq error is -17.9944/10/10
-meep: Frequency doesn't converge properly with a.
-FAIL 2D_convergence (exit status: 1)
+Shifted freq error is 0.127421/10/10

please investigate, fix and/or coordinate with upstream devs