Improve failures readability when testing exprs

[jhchabran]

With a bunch of t.Helpers() and line numbering, it's now much better. Because the originating file path (the sql) is absolute, you can click on it within your editor to open it (ignore the truncated version in the example below, it's a proper path in the code).

--- FAIL: TestMathFunctions (0.00s)
    --- FAIL: TestMathFunctions/math.floor (0.00s)
        --- FAIL: TestMathFunctions/math.floor/OK_@#math.floor(2.3) (0.00s)
            math_test.go:11:
                        Error Trace:    expr.go:160
                        Error:          Received unexpected error:
                                        found @, expected  at line 1, char 1
                        Test:           TestMathFunctions/math.floor/OK_@#math.floor(2.3)
                        Messages:       parse error at (/home/...)/testdata/math_functions.sql:2
                                        `@#math.floor(2.3)`
    --- FAIL: TestMathFunctions/math.abs (0.00s)
        --- FAIL: TestMathFunctions/math.abs/NOK_math.abs('foo') (0.00s)
            math_test.go:11:
                        Error Trace:    expr.go:180
                        Error:          Expect "cannot cast "foo" as double: strconv.ParseFloat: parsing "foo": invalid syntax" to match "cwfpwannot cast "foo" as double"
                        Test:           TestMathFunctions/math.abs/NOK_math.abs('foo')
                        Messages:       expected error message to match at (/home/...)/testdata/math_functions.sql:19

[asdine]

Awesome!