Enable multiline matching for runtime test regex

Currently the output is treated as one line with newline characters in
it, which breaks the typical use of the `^` and `$` character. E.g.

```
RUN bpftrace -e 'i:ms:1 { printf("%d\n", 1); exit(); }'
EXPECT ^[0-9]$
```

Will not work as the output contains "attaching 1 probe"

A test like:

```
RUN bpftrace -e 'i:ms:1 { printf("fff\n"); exit(); }'
EXPECT [0-9]
```

Doesn't fail, due to the probe count output.

With the `multiline` option set each the `^` and `$` will also match the
beginning and end of each line. Making the expect behave more like
expected.

tests/utils.py

@@ -115,7 +115,7 @@ def run_test(test):
             output += p.communicate()[0].decode()
 
             signal.alarm(0)
-            result = re.search(test.expect, output)
+            result = re.search(test.expect, output, re.M)
 
         except (TimeoutError):
             # Give it a last chance, the test might have worked but the