add pattern matching expect.find() for unit tests

settled on `find()` since the string method of the same name has the
same semantics. other options: `str_find()` since it fails on non-string
or `match()` (but that might imply that the pattern needs to match the
entire string).

library/lua/test_util/expect.lua

@@ -47,6 +47,14 @@ function expect.ge(a, b, comment)
     return a >= b, comment, ('%s < %s'):format(a, b)
 end
 
+function expect.find(pattern, str_to_match, comment)
+    if type(str_to_match) ~= 'string' then
+        return false, comment, 'expected string, got ' .. type(str_to_match)
+    end
+    return str_to_match:find(pattern), comment,
+            ('pattern "%s" not matched in "%s"'):format(pattern, str_to_match)
+end
+
 local function table_eq_recurse(a, b, keys, known_eq)
     if a == b then return true end
     local checked = {}

test/library/test_util/expect_unit.lua

@@ -1,5 +1,22 @@
 local expect_raw = require('test_util.expect')
 
+function test.find()
+    expect.true_(expect_raw.find('a ', 'a str', 'a comment'))
+
+    local ok, comment, msg = expect_raw.find('ab', 'a str', 'a comment')
+    expect.false_(ok)
+    expect.eq('a comment', comment)
+    expect.eq('pattern "ab" not matched in "a str"', msg)
+
+    ok, _, msg = expect_raw.find('pattern', nil)
+    expect.false_(ok)
+    expect.eq('expected string, got nil', msg)
+
+    ok, _, msg = expect_raw.find('pattern', {})
+    expect.false_(ok)
+    expect.eq('expected string, got table', msg)
+end
+
 function test.table_eq()
     expect.true_(expect_raw.table_eq({}, {}))
     expect.true_(expect_raw.table_eq({'a'}, {'a'}))