Here's what I want:
conn_config="cache_size=50M,eviction=(threads_max=4),statistics=(fast),statistics_log=(sources=("table:"),wait=10)"

It is the quoting around sources=("table:") that is the problem.

With no quotes (as shown above) I get:
WiredTiger Error: Invalid argument: Error parsing 'conn_config="cache_size=50M,eviction=(threads_max=4),statistics=(fast),statistics_log=(sources=("table:"),wait=10)"' at offset 97: New value starts without a separator

With single or triple backslashed quotes ...sources=(\"table:\"),wait=10)... or sources=(\\\"table:\\\"),wait=10)...
I get the exact same error for both:
{{wtperf: Error: unknown option '),wait=10)'
Options: <long list of all wtperf config options>}}