Merge #808: Exhaustive test improvements + exhaustive schnorrsig tests

Merge #808: Exhaustive test improvements + exhaustive schnorrsig tests

8b7dcdd955a4f57174f478e36bdae5b84784fb9c Add exhaustive test for extrakeys and schnorrsig (Pieter Wuille)
08d7d89299a6492bf9388b4662b709d268c8ea29 Make pubkey parsing test whether points are in the correct subgroup (Pieter Wuille)
87af00b511f2938b6b4799f94d446a005730515e Abstract out challenge computation in schnorrsig (Pieter Wuille)
63e1b2aa7d396209aa5e26aa540d9593ede312a6 Disable output buffering in tests_exhaustive.c (Pieter Wuille)
39f67dd072fc44c7c0d27b95610ba8912de56db5 Support splitting exhaustive tests across cores (Pieter Wuille)
e99b26fcd54cb4096515ba80cf0f79d147b2683c Give exhaustive_tests count and seed cmdline inputs (Pieter Wuille)
49e6630bca5f6628bd1fd92d70d465273d4d873f refactor: move RNG seeding to testrand (Pieter Wuille)
b110c106fa9704e30f6b0c2ffa6a2697031e89a8 Change exhaustive test groups so they have a point with X=1 (Pieter Wuille)
cec7b18a34e68adb04f31a71a2eb4c5fc97674ce Select exhaustive lambda in function of order (Pieter Wuille)
78f6cdfaae9866694dcb0eee966332688753a8c3 Make the curve B constant a secp256k1_fe (Pieter Wuille)
d7f39ae4b67ea1ac6f085e6262a5f53afc0c5a25 Delete gej_is_valid_var: unused outside tests (Pieter Wuille)
8bcd78cd791fd9209d72d6bce455c8d3cf2c0249 Make secp256k1_scalar_b32 detect overflow in scalar_low (Pieter Wuille)
c498366e5b2d9c60e2e677949cf7373dbe877515 Move exhaustive tests for recovery to module (Pieter Wuille)
be317915436909573733afe3972a9abdee9357f7 Make group order purely compile-time in exhaustive tests (Pieter Wuille)

Pull request description:

  A few miscellaneous improvements:
  * Just use EXHAUSTIVE_TEST_ORDER as order everywhere, rather than a variable
  * Move exhaustive tests for recovery module to the recovery module directory
  * Make `secp256k1_scalar_set_b32` detect overflow correctly for scalar_low (a comment in the recovery exhaustive test indicated why this was the case, but this looks incorrect).
  * Change the small test groups so that they include a point with X coordinate 1.
  * Initialize the RNG seed, allowing configurating from the cmdline, and report it.
  * Permit changing the number of iterations (re-randomizing for each).
  * Support splitting the work across cores from the cmdline.

  And a big one:
  * Add exhaustive tests for schnorrsig module (and limited ones for extrakeys).

ACKs for top commit:
  real-or-random:
    ACK 8b7dcdd955a4f57174f478e36bdae5b84784fb9c
  jonasnick:
    ACK 8b7dcdd955a4f57174f478e36bdae5b84784fb9c

Tree-SHA512: 18d7f362402085238faaced164c0ca34079717a477001fc0b13448b3529ea2ad705793a13b7a36f34bf12e9231fee11070f88cc51bfc2a83ca82aa13f7aaae71