[ubsan][test] Be more specific than "error:" with implicit-check-not

The string "error:" is too general and reports false positive from system
messages that can occur on stderr when using network devices for testing.
The string "runtime error:" is sufficiently unique for these connection
errors to pass through w/o a false positive.

rdar://100564373

Reviewed By: thetruestblue

Differential Revision: https://reviews.llvm.org/D139230