Add benchmark for ImapParserTest::benchParseQuotedString

The results on my machine are in RelWithDebInfo mode:

********* Start testing of ImapParserTest *********
Config: Using QtTest library 5.7.1, Qt 5.7.1 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 6.3.1 20170109)
PASS   : ImapParserTest::initTestCase()
PASS   : ImapParserTest::benchParseQuotedString(quoted)
RESULT : ImapParserTest::benchParseQuotedString():"quoted":
     0.000131 msecs per iteration (total: 131, iterations: 1000000)
     468.054021 CPU cycles per iteration (total: 468,054,022, iterations: 1000000)
     1,193.332691 instructions per iteration (total: 1,193,332,692, iterations: 1000000)
PASS   : ImapParserTest::benchParseQuotedString(unquoted)
RESULT : ImapParserTest::benchParseQuotedString():"unquoted":
     0.000086 msecs per iteration (total: 86, iterations: 1000000)
     297.429136 CPU cycles per iteration (total: 297,429,136, iterations: 1000000)
     605.241426 instructions per iteration (total: 605,241,426, iterations: 1000000)
PASS   : ImapParserTest::cleanupTestCase()
Totals: 4 passed, 0 failed, 0 skipped, 0 blacklisted, 437ms
********* Finished testing of ImapParserTest *********

With heaptrack, I see the following results:

calls to allocation functions:
    19318035 (75216/s)
temporary allocations:
    3304278 (17.1%, 12865/s)
bytes allocated in total (ignoring deallocations):
    1.85 GiB (7.4 MiB/s)