Ignore negative counts when generating coverage info

For some reason generating the coverage report yields a negative count
somewhere in `PersistentVariables.ipp`. I don't know why this happens.
Compiling with `-fprofile-update=atomic` as suggested by the error
message doesn't help. It only makes the SPI supervisor test require ~100
bytes more stack without getting rid of the error. Therefore, I followed
the second suggestion and changed the coverage command to ignore such
negative counts.

cmake/coverage.cmake

@@ -1,8 +1,9 @@
 # ---- Variables ----
 
 # We use variables separate from what CTest uses, because those have customization issues
-set(COVERAGE_TRACE_COMMAND lcov -c -q -o "${PROJECT_BINARY_DIR}/coverage.info" -d
-                           "${PROJECT_BINARY_DIR}" --include "${PROJECT_SOURCE_DIR}/*"
+set(COVERAGE_TRACE_COMMAND
+    lcov -c -q -o "${PROJECT_BINARY_DIR}/coverage.info" -d "${PROJECT_BINARY_DIR}" --include
+    "${PROJECT_SOURCE_DIR}/*" --ignore-errors negative
     CACHE STRING "; separated command to generate a trace for the 'coverage' target"
 )