Recalibrate unit tests

[sipa]

The tests (src/tests.cpp and included modules) consist of some tests whose runtime is independent of the TEST_ITERS variable, and some which are. The ones that are independent take a significant amount of time (compared to the other ones, at a normal TEST_ITERS level). The ones that do depend on it do so in a rather inconsistent way.

So to improve upon that:

• Turn constant-time tests into TEST_ITERS-proportional ones were possible.
• Perhaps disable some very slow constant-time ones entirely at very low TEST_ITERS levels.
• Run benchmarks on each of the proportional ones, and adjust to make sure they all get a somewhat representative amount of runtime in typical configurations.

[real-or-random]

gprof of ./tests 1 shows that the new test_ecmult_constants() tests is indeed very heavy.

gprof-tests1.txt

[real-or-random]

Related comment introduced in 17065f4:

secp256k1/src/tests.c

Lines 7091 to 7093 in 3efeb9d

	/* Randomize the context only with probability 15/16
	to make sure we test without context randomization from time to time.
	TODO Reconsider this when recalibrating the tests. */