amp-pixel: Minor test improvements

[mdmower]

Resolve sinon warnings:

ERROR: 'The <amp-pixel> src attribute must start with "https://" or "//". Invalid value: http://pubads.g.doubleclick.net/activity;dc_iu=1/abc;ord=2'
    The test "amp-pixel   should disallow http URLs" resulted in a call to console.error. (See above line.)
    ⤷ If the error is not expected, fix the code that generated the error.
    ⤷ If the error is expected (and synchronous), use the following pattern to wrap the test code that generated the error:
        'allowConsoleError(() => { <code that generated the error> });'
    ⤷ If the error is expected (and asynchronous), use the following pattern at the top of the test:
        'expectAsyncConsoleError(<string or regex>[, <number of times the error message repeats>]);'
ERROR: 'The <amp-pixel> src attribute must start with "https://" or "//". Invalid value: /activity;dc_iu=1/abc;ord=2'
    The test "amp-pixel   should disallow relative URLs" resulted in a call to console.error. (See above line.)
    ⤷ If the error is not expected, fix the code that generated the error.
    ⤷ If the error is expected (and synchronous), use the following pattern to wrap the test code that generated the error:
        'allowConsoleError(() => { <code that generated the error> });'
    ⤷ If the error is expected (and asynchronous), use the following pattern at the top of the test:
        'expectAsyncConsoleError(<string or regex>[, <number of times the error message repeats>]);'
ERROR: 'The <amp-pixel> src attribute must start with "https://" or "//". Invalid value: https/activity;dc_iu=1/abc;ord=2'
    The test "amp-pixel   should disallow fake-protocol URLs" resulted in a call to console.error. (See above line.)
    ⤷ If the error is not expected, fix the code that generated the error.
    ⤷ If the error is expected (and synchronous), use the following pattern to wrap the test code that generated the error:
        'allowConsoleError(() => { <code that generated the error> });'
    ⤷ If the error is expected (and asynchronous), use the following pattern at the top of the test:
        'expectAsyncConsoleError(<string or regex>[, <number of times the error message repeats>]);'

Full log: https://gist.github.com/mdmower/d2252c6dc06d096687c56a6368a30c41

[kristoferbaxter]

Nit, can reuse the rejectedWith content by creating a single const reference.

[mdmower]

Thanks for the review. This change has been made.

[kristoferbaxter]

LGTM, might want to consider the nit.