Bug 1527461 [wpt PR 15353] - Add .mailmap to map people to their current names, a=testonly

Automatic update from web-platform-tests
Add .mailmap to map people to their current names

Note that this isn't used by `git log` by default (as of 2.20.1): that
depends on log.mailmap being set to true.

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Add Tess to .mailmap

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wpt-commits: 39f54a3dd48b560289e0f455308acf7d32fad043, f8a1bfbe5454352d3f5b58845829968ff212519b
wpt-pr: 15353

testing/web-platform/tests/.mailmap

@@ -0,0 +1,9 @@
+# People who've changed name:
+
+# Sam Sneddon:
+Sam Sneddon <me@gsnedders.com>
+Sam Sneddon <me@gsnedders.com> <geoffers@gmail.com>
+
+# Theresa O'Connor:
+Theresa O'Connor <eoconnor@apple.com>
+Theresa O'Connor <hober0@gmail.com>