logictest: skip alter_table under race

We just saw a test run where `alter_table` took more than an hour under
race. None of the metamorphic variables seem likely to be the cause of
the slowdown, so probably just the sheer number of schema changes in
this file are to blame.

Release note: None

pkg/sql/logictest/testdata/logic_test/alter_table

@@ -1,3 +1,6 @@
+# We've seen this file take more than an hour under race.
+skip under race
+
 statement ok
 CREATE TABLE other (b INT PRIMARY KEY)