tests/bg: Don't require exact match for bg output

The format of the command string displayed by the bg built-in is not
strictly specified by POSIX. ksh seems to print the entire command line
which may include the "&" symbol and other commands that are not part of
the job. Probably we should accept this as conformant.

tests/bg-p.tst

@@ -72,7 +72,7 @@ __OUT__
 test_OE -e 0 'bg prints resumed job' -m
 sleep 1&
 bg >bg.out
-grep -qx '\[[[:digit:]][[:digit:]]*][[:blank:]]*sleep 1' bg.out
+grep -q '^\[[[:digit:]][[:digit:]]*][[:blank:]]*sleep 1' bg.out
 __IN__
 
 test_OE -e 0 'exit status of bg' -m